The Block System and its History

Contents

1. Introduction
2. Fixed Signals
3. Before Telecommunications
4. The Clayton Tunnel Accident
5. The First Block Systems
6. Controlled Manual Block
7. References

Railways are different from all other transport systems in that the routes of the trains are determined by the road, not by the driver; trains cannot take evasive action or move to avoid one another. Speed cannot be reduced abruptly and stops cannot usually be made in the distance visible to the driver when the train is moving at normal speed. Therefore, special arrangements are necessary to avoid conflicts. Opposing trains must come together only where there are facilities for guiding one around the other (called "crossing" in Britain and France, "meeting" in America). Trains in the same direction must not encounter one another unexpectedly. Of course, stationary equipment or other obstacles must not occupy the track.

In this article, "America" is generally used instead of "United States," but it is quite appropriate if "North America" is understood, as Canada and Mexico both adopted United States practices very closely. "Britain" comprises, of course, England and Scotland, but should be understood to include Ireland as well. British and American practices, although superficially quite distinct, are really very closely related and represent evolution to suit local conditions. The person who operates a train is called a "driver" (America, engineman; France, machiniste; Germany, führer) for simplicity. Mixed British and American terminology is used here, hoping to help readers become familiar with both.

Three eras in railway operation can be distinguished by the means of communication available. In the first era, before 1850, there were no telecommunications of any kind generally available. Two points beyond shouting or gesticulating range could only communicate by written messages carried from one point to the other. In the second era, from 1850 to 1950, wire communications were available, by telegraph or telephone, which was instantaneous and of effectively unlimited range. However, communication with the trains themselves was not possible. Since 1950, radio has made it possible to communicate directly with the drivers of trains, and more recently, to determine the location of trains directly (GPS).

Although optical telegraphs, originated in 1794 by the Chappe brothers in France, were still in use at the time of the first railways, they were never used in railway operation except in the most elementary way, and never as the rapid telecommunications they represent. They were not only expensive, but useless in bad weather. The optical telegraph, the sémaphore, gave its name to the railway signal derived from it. Tidal ball signals were pressed into railway service to a limited degree. A typical ball signal (B&O) is shown at the right. The colour of the balls is assumed; they were commonly white or black as well. If used at a crossing, the indications might be, two balls: proceed on road A; one ball: proceed on road B; no balls: stop on both lines. Common-road practice largely provided the lamps that were used on railways, instead of the better nautical practice.

Essential to railway operation are the common mobile signals (a very useful French term, absent in English) represented by hand, flag and lamp signals, flags and lamps carried by trains, locomotive whistle signals and communicating signals. Although important, these are often neglected in discussions of operation and signalling.

Signals are the means of local communication between trains and the wayside. Fixed signals, in particular, are devices of fixed location communicating primarily with drivers. Drivers communicate with wayside employes, and other trains, by means of whistle signals, and hand, flag and lamp signals are used between employees. Pyrotechnic fusees supplement lamp signals. Detonators (America, torpedoes; France, pétards) are acoustical signals also supplementing lamp signals, fixed on the rail and exploding when run over. Flag signals are used by day, lamp signals by night. Hand signals are given by the arms and hands, as well as with flags or lamps.

Signal engineers use the words "advance" and "rear" in special ways to express the location of a point relative to a signal. As you approach the signal, "in advance" is beyond the signal, while "in rear" is where you are. The common usage is almost the opposite, so care should be taken. In French, the technical words are "en aval" and "en amont", meaning downhill and uphill in the direction a river flows, which is very vivid and unambiguous. If a sign is placed 100 yards in rear of a signal, you pass this sign before reaching the signal. A signal may protect a train in advance of it. The front of a signal is seen from in its rear!

What a signal, especially a fixed signal, displays is its aspect; what the aspect means is the indication. Aspects may be given by colour, shape or motion. Colour is best used in the dark, when even the light of a simple oil lamp is quite visible. The bare flame, or "white," is by far the most visible. A red lens produces a much dimmer red light, but the colour is distinct and visible under most conditions. A green lens produces a light even less bright, and in early days the colour was quite variable between different glasses, from a yellowish green to a bluish green. Nevertheless, it is easily distinguished from white or red. Blue and violet glasses transmitted so little intensity that they could be used only over short distances. Blue and yellow glasses were easily confused with some greens, and deeper yellows with red. The Chappe brothers found in their semaphore experments that if the visibility of a white light was 1, then the visibility of a red light was 1/3, a green light 1/5, and a blue light 1/7. For these reasons, it was agreed on 19 January 1841 at the Conference of Railway Chairmen and Managers in Birmingham that only white, green and red lamps would be used to give signals. White was the colour of safety, green of caution, and red of danger, displayed either by a flag or a lamp. A white light meant proceed, a green light reduce speed, go slow, and a red light stop. These colours were generally adopted, not only in Britain, but in America and Europe as well. In America, blue was often used instead of green for caution.

Coloured lights have been enormously improved over the years, but the improvement was remarkably slow. Not only were burners primitive (the more efficient and brighter Argand burners with Welsbach mantles were rarely or never used in railway signals until recent years), but the coloured glass was, in general, also inadequate. Electric lighting solved the intensity problem, and glass manufacturers began to make better signal colours, only in the 20th century. With electric lamps and good lenses, colour lights can now be used by daylight as well as by night, and are, in fact, the most common sort of railway signals now in use. Throughout the 19th century, however, round or flat wicks burning mineral (paraffin) or vegetable (colza) oil were universal. Paraffin generally replaced colza oil around 1885.

Although attractive, coloured targets by day are far less distinctive than coloured lights by night. (A target, or vane, is a flat panel.) Any target shaded in bright sunlight can appear black, whatever its actual colour. Green is a particularly poor choice for a target, since it blends into its background under most lighting conditions. In Belgium, green flags were found so poor that white flags replaced them for signalling, until 1920. Only when viewing a coloured target with the sun to one's back is the colour distinct and unmistakable. This was recognized quite early in British practice, and reliance on colour alone by day was firmly rejected. The idea of a target rotating a half-revolution with different colours on the two sides was replaced by targets of different shapes at 90° and a rotation of a quarter-revolution. The rotating coloured vane was the typical American fixed signal before 1870, though later quite forgotten. Such signals were used in the Reading's towers, for example. An excellent example of the use of form is the disc-and-crossbar signal of the GWR. When turned to face the driver, the disc indicated safety, while the crossbar indicated danger. Both were painted bright red. The colour was, as Brunel said, completely immaterial. Indeed, against the sky both appeared black. The most effective use of form is in the semaphore, where the contrast is between the vertical mast and the arm, either horizontal or inclined (best making an acute angle with the mast). This form can be seen and recognized from a great distance, much better than the shape of a target. Until the introduction of electric lighting, daytime form signals, such as semaphores, were always much more distinctive than light signals.

French signals were typically targets presenting their shapes frontally to the driver in the most restrictive aspect, and turned on edge (so they could not be seen) for the most permissive aspect. The French Signal Code of 1885 states explicity that: "Dans tous les cas, l'absence de signal indique que la voie est libre." This is anathema to British or American practice. Indeed, the American Standard Code of Operating Rules has Rule 27: "A signal imperfectly displayed, or the absence of a signal where a signal is usually shown, must be regarded as the most restrictive indication that can be given by that signal..." In general, British and American practice demand a positive proceed signal, which has been demonstrated to be of great importance to good operation, drivers constantly looking for assurances to proceed at normal speed, and hesitant when such assurances are not given.

The method of electric bells originated in Austria-Hungary in the 1850's, and spread to several European systems, though never to Britain or America. The idea was to put large gongs at the stations at each end of a section, as well as spaced along the line, at level crossings and other points, with the aim that they could be heard everywhere. Gong signals announced the departure of trains, and no other train was then allowed on the line. Stations communicated by telegraph or telephone as well, coordinating the dispatch of trains. In case of emergency or error, all trains on the line could be stopped quickly, until the problem was sorted out. This was a relatively inexpensive system that could manage a heavy traffic. In Britain, and especially in America, there were too few employees regularly along the line to make bells practical. In France, electric bells were used on the P.-L.-M., P.-O., État and Ouest beginning in the 1880's.

There would be little disagreement that Britain invented the block system and interlocking (in addition, of course, to the modern railway itself), and the United States the closed track circuit and the automatic block it made possible, as well as the centralized control of movments (the dispatcher system). Railway operations, however, assumed different flavours in different countries, with the creation of national characteristics, often mutually incomprehensible. One can recognize British, American, German, French and Belgian flavours, and blendings of them in Spain and Italy. The Netherlands, Austria and Switzerland generally followed German practices.

The Liverpool and Manchester, regarded as the first modern high-speed steam railway, was opened in 1830. The electromagnetic telegraph appeared in England in 1838 as the Coole and Wheatstone 5-needle hatchment telegraph. Experiments with electrical communication were taking place in Germany at the same time, by Gauss, Weber and Steinheil, and in connection with railways. These experiments were less well known in Britain and America, and histories neglect them. The superiority of aerial conductors on insulators, and the use of the rails and the earth as conductors were some of the discoveries made. Telegraphs did not become general on railways in Britain until around 1852, and even later in the United States, where the Morse patents and poorly-constructed lines hindered developments. Even here, the first use of telegraphic messages to move trains was in 1851, and after railway companies controlled their own lines telegraphs came into widespread use. Nevertheless, from 1830 to 1850 railways had to be operated without telecommunications and adequate methods were developed.

Let's first discuss single track, where trains must operated in both directions. Single track was rare in Britain, but elsewhere in the world it was frequently found. In America, single track was almost universal for many years. The simplest method to ensure safety is to allow only "one engine in steam," as it is known in Britain. In France, this was called exploitation en navette; a navette is a weaver's shuttle, that passes back and forth in the shed. The train need not worry about any other, and "protection" is not required. However, it is appropriate only for rather short lines, and especially those that have access only from one end (dead-end or cul-de-sac branches). For such lines, it is very economical. This method was never formally employed in America, even on lines where it would have been quite appropriate, but was widely used elsewhere in the world.

If one train is not enough, or trains enter and leave at both ends, the train staff, (French, bâton-pilote) is a very safe alternative. A unique physical token, clearly identified, serves as authority to occupy the track in the section; only one such token exists, so only one train can ever occupy the section. If trains must follow one another without trains in the opposite direction, the drivers of preceding trains are shown the train staff (and may prepare a ticket as evidence of having seen it), while the last train carries the staff. There is obviously the problem of having the train staff at the wrong end of the section. Often, only carrying the train staff on horseback to the other end was the solution. Careful planning can eliminate most problems, but is not always convenient. Sometimes, an actual person, a pilotman, can take the place of a train staff, especially in emergencies. It was quite difficult to operate ballast trains and the like in train staff territory. Some managers called it a way to ensure safety by eliminating railway travel. Although praised by Mark Huish of the L&NWR, most managers strongly opposed the train staff. Nevertheless, the Board of Trade enforced it as almost the only way to operate single lines in Britain. Most other countries just used the telegraph with no train staffs. In France, the train staff was used only on the 12.9 km line of the Ouest between Verneuil and La Loupe, with four intermediate stations (5 sections). The train staff was made less onerous in the days of telecommunications by the development of the small token to replace the staff, and of interlocked electrical machines so that a token was available at any point where it was needed.

The fundamental tool for operating single track was the time table. On single track, it specified the stations at which trains would "meet" or "cross". Each train had to proceed to the crossing point, and wait indefinitely for the opposing train. Obviously, delay to a train would have serious effects that would cascade to other trains, delaying them in turn. For short distances, and light traffic, delays were usually tolerable. In a pinch, a train could proceed behind a flag at a walking pace, perhaps to find the opposing train disabled ahead. When trains began to cover relatively great distances, as in America, the delays could be intolerable.

The solution to this was to classify trains by importance. At least two classes were usually necessary, passenger trains and freight trains, since delays to passengers were much less tolerable than delays to uncomplaining freight. Then, passenger trains would not wait at crossing points with freight trains. Freight trains had to clear the time table time of passenger trains and wait until they had passed. Of course, freight trains had to wait for opposing trains at crossing points with other freight trains, and passenger trains had to do the same with respect to passenger trains. Just this much was a great relief, particularly when there were only a couple of passenger trains each day and they usually ran close to time. Trains of a higher class were said to be superior to trains of lower classes, who had to stay out of their way. The classes were numbered first, second and so on. In general, a first class train was a passenger train; a second class train was a through freight, and a third class train was a local freight.

The next step was to make trains in one direction superior to trains of the same class in the opposite direction. Sometimes a wait of a specified length of time at the time table crossing point was required, but more usually the inferior train had to clear the time of the superior train if it could not reach the time table crossing point in time. This meant that first class trains in the favoured direction could move regardless of other trains, and those in the other direction had only to avoid these trains.

So far, the improvements to basic time table operation are simple and safe, and were retained as long as time table operation was used in the United States, where indeed it was standard. Additional rules were made in an attempt to reduce delays, as when an important train was disabled at some point, tying up everything in front of it. A superior train would lose its rights after it was delayed a certain amount, perhaps a half hour or an hour, and then had to proceed keeping clear of the trains that previously kept clear of it. This gave rise to complex calculations by crews on the road, that were not always decided correctly. It was the foundation of the American tradition that placed the burden of deciding whether to proceed or not on the train crews, on the basis of the time table that was the authority for movement.

The time table is very cumbersome with respect to extra trains, those not given schedules. A written message can be carried by train from end to end of the line if the need for the extra train is known sufficiently in advance. If there is not time for this, a preceding train can carry a flag on the engine to show that an extra train is following. If the flag were red, the following train had the same rights as the train carrying the flag, and would be in modern terms a "section" of the schedule. A white flag indicated that the extra train would move keeping clear of opposing regular trains. What opposing extra trains would do had to be decided when white flags met on the road. There were also humble ballast and wood trains in America that would run the best they could.

Time table operation worked more or less satisfactorily, and was given up with reluctance. Many of its faults and inefficiencies were overcome by the appointment of a dispatcher for each division of the railway, responsible for the central direction of the movement of trains. This, however, required the resources of the telegraph, which was not available until about 1855. Andrew Carnegie was the first dispatcher on the Pittsburgh Division of the Pennsylvania Railroad, in 1854. The dispatcher could change crossing points, and arrange for the running of extra trains, completely eliminating time table delays. The dispatcher system not ony flourished in America, where it became universal and was greatly elaborated, but was also used by the London and South Western Railway on its single lines to the West.

Double track operation is very much simpler than single track operation. Opposing trains do not exist, so long as every train keeps to its proper track of assigned direction. In Britain, France and Belgium trains keep to the left, while in German and American practice they keep to the right. The earliest locomotives in all countries were driven from the right, as wagons had been, so running on the left-hand track was a perverse choice, and grew worse as locomotives became larger. This practice seems simply to have been in imitation of road traffic, and was given little thought until it became ingrained. Signals were placed to the left of the track, with semaphore arms pointing away from the track (so trains would not strike them). In Britain, all companies but the GWR eventually moved drivers to the left, as was done in France and Belgium. The Germans and Americans logically just drove trains on the right-hand track, keeping the driver where he was. Signals were then placed on the right, with arms still pointing away from the track. Road traffic, oddly, later conformed to this, with the driver moved to the left-hand side.

The time table serves only to dispatch trains at convenient intervals. An extra train can be interpolated with no special effort. As on single track, it is still necessary for a train to protect its rear at points were it can be run into by a following train moving at speed. This is done by a guard running back with a red flag, red lantern, and detonators. It requires about five minutes to get back the necessary distance, so this minimum time interval must be maintained between trains. Quite generally, 10 minutes has been accepted as the minimum time between trains. This time interval is maintained by scheduling trains 10 minutes or more apart, by dispatching faster trains in front of slower, and by signals from stations and trackside employees. This system has worked surprisingly well, but it is not foolproof. The time interval can be eroded by slow running, or flagmen may not go back promptly enough. There is always the judgment of whether to go back or not, since there is always a considerable penalty in time when the full flagging ritual is carried out, and the danger of a flagman's being left behind.

The original idea of posting "policemen" along the line to maintain the time interval in general required too much staffing. Policemen, in their shelters, soon became concentrated at level crossings, tunnel mouths and other critical places, rather than being spaced more or less evenly. In the late 1840's they acquired the telegraph to operate isolated blocks, as at tunnels, and handled fixed signals. This is clearly illustrated in the Bourne engraving of the early GWR at Box Tunnel, with policeman, his shelter, disc-and-crossbar, and "fantail" for spacing trains. These policemen evolved into the signalmen in their elevated signal boxes. In America, the Philadelphia and Reading put watchmen in octagonal lighthouse-like structures 30 to 50 ft high at blind curves. They had red, white and blue vanes on top, rotating about a vertical axis, for signals, illuminated by night. The telegraph was used for tunnel blocks. These "towers" provided the term popularly used in America instead of "signalbox," though the official term was often "signal cabin." The "police" system did not long survive, and the duty of protecting trains devolved on their crews instead.

In America, the time interval system was embodied in the Standard Code operating rules 91 and 99. Rule 91 stated: "Unless some form of block system is used, trains in the same direction must keep not less than ten minutes apart, except in closing up at stations." This is easier said than done, of course, and meant that employes along the line should warn trains following too closely, though no regular means of doing so was available, except for train order operators with signals. Holding up a watch and pointing ahead was an informal means of observing Rule 91. Rule 99 had various forms. The "short" form left the details up to the experience of the flagman, and merely prescribed that he should provide protection whenever a train was in danger of being overtaken. The "long" form specified in detail distances and procedures, attempting to take into account such factors as gradients, weather and so forth. It was impossible to frame a completely satisfactory rule, and the "short" form was generally preferred. Through these rules, the time interval system remained a feature of American train operation until the latest times. It must be realized that with light traffic, which typifies American operation, this system is practical and economical. It was far too expensive to maintain block posts at reasonable intervals for a heavy traffic. Where manual block systems did exist, the signalmen were usually the train order telegraph operators, or towermen at crossings and junctions, so that blocks were commonly from 15 to 25 miles long.

The block system originated in Britain in the 1850's. By the time of the Regulation of Railways Act of 1889, which prescribed "lock, block and brake," it was quite general. It was typical of British railway legislation that it merely reflected what was considered the best practice and enforced it uniformly, without a great deal of compulsion. The manual block was introduced by the Pennsylvania Railroad in 1876, and soon covered all its main lines, but was not imitated by other companies. Automatic block systems became popular after 1895, and were soon widely used. Most manual block systems were installed around this time, reflecting the general interest in block systems that had arisen. Often, they applied only to passenger trains. In America, the Interstate Commerce Commission tried to compel its generally half-baked ideas and compel the block system in the years 1900-1920, with very little effect. It could not legally require the block system, and had to confine its efforts to uncoordinated and clumsy interference that had little practical effect on safety, but was quite annoying.

On 29 December 1871, it was required by law in Germany that only one train could be admitted to the line between two stations. This was, of course, a block system, and reflected the importance of stations in railway operation. After 27 October 1875 the block system was required in Holland. This gives an idea of the dates of adoption of the block system in Europe. In many cases, as in France, the time interval system was abandoned only with reluctance.

The first telegraph blocks worked by the methods suggested by Cooke protected tunnels, ensuring that a train entering the tunnel had exited before a second was admitted. There was a great fear of collision in tunnels at the time that prompted these installations. The first may have been Wickwar Tunnel on the Bristol and Gloucester in 1845. In 1846, the Midland protected Thackley, Cleugh and Duffield tunnels in Derbyshire. These early blocks were probably provided with a single-needle instrument at each end of the tunnel, and signalling bells for calling attention. The single-needle instruments could be used for conversing. "Train In" was signalled by a long deflection to the left, "train out" by a long deflection to the right. One wire was used for both lines.

The Electric Telegraph Company, a contractor with the Great Western from 1845, provided Nott's telegraphs for signalling Box Tunnel in December, 1847. The Nott telegraph was a dial instrument in which the needle at one end followed the motions of the needle at the other. There were at least 20 positions, each corresponding to a message. This telegraph was often out of order, and was apparently replaced by more conventional instruments in 1852.

In 1848 the Midland established telegraph blocks at the tunnels between Ambergate and Rowsley on the new line to Manchester. Also in 1848, the Eastern Counties Railway adopted the 5-needle Cooke and Wheatstone telegraph for the single line between Norwich and Yarmouth, with one needle for each station. This was the beginning of continuous blocks, instead of just isolated blocks.

Clayton tunnel is about 5 miles north of Brighton on the main line of the London, Brighton and South Coast Railway. It is 2259 yards long, piercing the South Downs. The summit of the gradient is at the south portal. A single-needle telegraph and signalling bells were installed in 1851. In 1853, C. F. Whitworth patented an automatic signal for protecting tunnels on 28 January. This signal was a spectacle, or double-disc, signal rotating about a horizontal axis. A passing train operated a treadle that set the signal to Danger. If the signal was passed at Danger, a gong was rung to warn the engineman. When "train out" was sent to the signalman at the entrance, the signal was reset to Clear. This signal was used on the Lancashire and Yorkshire at several locations. It was installed at Clayton tunnel sometime in the 50's.

A serious accident occurred just within its south portal on 25 August 1861, when the third of three closely-spaced trains ran into the second, which was reversing towards the south portal. Though this accident brought out the deficiencies in the telegraphic block as then used, the block was not the primary cause of the accident. The account given in Rolt contains some inconsistencies. The accident was investigated by Captain Tyler, who gave a detailed account.

On this bright August morning, three trains left Brighton at 8.28, 8.31 and 8.35. The first two were excursions, of 16 and 17 coaches, and the third was the regular train, of 12 coaches. The advertised times gave spacings of 10 and 15 minutes, but the actual spacings were dangerously small, and the principal cause of the accident. It was probably thought that the intervals would increase with distance, the regular train having to make stops. This illustrates the intensity of traffic handled under the time interval system.

The first train entered the tunnel, but did not operate the Whitworth signal. Signalman Killick gave "train in" to signalman Brown at the other portal. Brown probably repeated the "train in" and waited for the train to arrive. On the approach of the second train, signalman Killick grabbed a red flag to stop it, since the Whitworth signal was still showing Clear, but just as he left his cabin the second excursion entered the tunnel, unwarned by the faulty signal. Killick gave a second "train in" to Brown, who shortly gave a "train out" for the first excursion. Killick asked if the tunnel was clear, and Brown said it was, forgetting the unusual second "train in". Killick, apparently, then made no attmept to stop the third train, thinking the second train had safely exited.

None of this made any difference, since the driver of the second excursion had caught a glimpse of the red flag and had brought his train to a stop as soon as possible, thinking that something had happened to the preceding train. He unwisely backed the short distance to the south portal, but as he did so the regular train rushed into the tunnel and collided with the train ahead inside the tunnel, killing 23 passengers and injuring 176. For some reason a guard from the second train had not run back to flag the regular train; Killick apparently thought that the tunnel was clear. The failure of the automatic distant signal was one cause of the collision, but the interval between the trains was much too short for any effective action in an emergency.

The major defects of this kind of telegraph block were the reliance on momentary signals, and the use of one wire for both tracks. If a persistent signal of "train on line" was provided, there would necessarily have to be an instrument for each line. These improvements would not have prevented the accident, since Brown would have given "line clear" after the first train left the tunnel (as he did), although the second train was still in the tunnel. The culprit was neither the telegraph nor the Whitworth signal, but dangerous operating procedures combined with heavy traffic. Incidentally, a Whitworth signal had failed at Radcliffe Bridge on the L&Y in 1859.

W. F. Cooke suggested the telegraph block system in a pamphlet of 1842. By "system" we understand a series of blocks, rather than an isolated block at a tunnel or other short stretch of danger. The first system was established on the single-track Eastern Counties line between Norwich and Yarmouth in 1844, with three intermediate stations. Each station had a Cooke 5-needle telegraph showing the conditions at all stations. Trains were signalled from station to station, using "train in" and "train out" signals. This elaborate system was soon replaced by one with two single-needle instruments at each station, communicating with the stations on either side. The Great Western Railway used the telegraph between its Birmingham station and Hockley from the opening in 1845, soon extending it through the Snow Hill tunnel to Bordesley. Brunel called the attention of his Directors to the Cooke and Wheatstone telegraph, arranging for its trial between Paddington and West Drayton, 13 miles, in 1838. This was the original 5-needle hatchment telegraph with buried conductors. Double needle instruments with aerial wires were in use from Paddington to Slough in 1843. Von Weber reports that the telegraph was used on the single-track Northampton-Peterborough branch of the London and Birmingham Railway in 1844. This line was opened 31 May 1845. The early use of the telegraph on the Blackwall railway was for controlling their cable-operated cars, not for a block system.

The word "block" comes from Cooke's choice of this noun to describe a defined length of railway, perhaps as a block is a division of a piece of timber. Its etymology does not reinforce this concept well, and some have taken it as a verb, thinking of blocking trains from proceeding, or blocking the telegraph handle over to one position to another.

The earliest blocks used "speaking" telegraphs with one or two needles in a single case. Inside was a magnetized needle in a coil of wire that was deflected to one side or another by a current in the coil. The needles were from 2 to 6 inches long. A longer needle took longer to move, but retained its magnetization fairly well. Short needles moved more quickly, but had to be frequently remagnetized by stroking with a strong permanent magnet. The motion of the needle was communicated to an indicating needle on the front of the case, the one actually seen behind a glass screen. This was a delicate mechanism, but a sensitive one. Handles at the bottom of the case below each needle were moved right or left to deflect the local and distant needles correspondingly by operating a commutator, sending polarized signals. An unusually large current through the instrument might remagnetize the needle in the opposite direction, so that the needles at the two ends of the circuit moved oppositely. This was easily detected by observing the deflection caused by moving a handle; if it was reversed, then so was the magnetization of the needle. These unusually large currents were the result of strong earth currents, something that affects any electrical conductor suspended above the earth. These currents accompany thunderstorms and occasional magnetic storms caused by ionospheric currents.

When used for signalling, the telegraph needle showed "train on line" when deflected in one direction, "line clear" when deflected in the other. If no current was supplied, the needle was vertical. It was controlled by the man at the exit of a block, and normally showed "line clear", held in this position by "pegging" the operating handle in the appropriate position. When a train entered the block, the signalman at that end put his signals to stop, and notified the man at the exit end, who replied by pegging the instrument at "train on line". When the train left the block, the instrument was again pegged at "line clear", which was observed by the man at the entrance, who again put his signals at clear. In many early cases, the indications were given only momentarily, by holding the needle over for a few seconds. They were recorded in a book, with the time of transmission. Such recording is essential when temporary signals are used.

Some sources say that the first isolated tunnel block was established at Clayton tunnel in 1851. However, Box tunnel on the GWR had the telegraph in 1847. This was, apparently, a dial telegraph. The rules for its use can be found in MacDermot. In the early 50's, many tunnels and inclines were so equipped, such as the Lickey incline, the tunnel at the east end of New Street station in Birmingham, the Cowlairs incline in Glasgow, Sapperton (GWR), Kilsby (L&NWR), Wickwar (MR) and Stoke (GNR) tunnels, as well as the approaches to Victoria Station in London.

C. V. Walker, Telegraph Superintendent of the South Eastern Railway, suggested in 1851 the use of bells between Spa Road and London Bridge, 850 yards, to describe trains to the switchtender at the station, and to admit trains as desired, since Croydon, Brighton and South Eastern trains arrived on the same tracks. Spa Road is a now-vanished station, near what is now Tower Bridge Road, on the viaduct between London Bridge and South Bermondsey, and was the first railway station in London (1836). The first bells were placed in service in late January 1851. This system was afterwards greatly expanded, using the single-stroke bells to offer and accept trains between stations. By 1863, there were 330 bells covering 275 miles of the South Eastern's lines. 84 bells had counting indices to show the number of strokes. The ringing key was called a "pecker". The general code was one ding for an up train out, two for a down train out, and three for a train in. The bells were 4 or 5 inches in diameter across the mouth, and the magnets were wound with large wire. The bells were also used for other purposes, including the important duty of the description of trains. The general rule in the use of bells was already appreciated at this time, that the recipient should repeat the code back to the sender, to ensure that the signal has been properly understood. Each bell at a given office has a different tone, so they may be distinguished.

A curious bell circuit used batteries of opposing polarity at the two ends, so that the net current was zero. If one or the other battery was shorted out, then the bells rang. They also rang if any point on the circuit was earthed, so that the bells could be rung in case of trouble on the line without having to have a battery available.

These bells survived as the signalling bells later used for communication between signal boxes. However, the Walker system had no visual indicator of line occupancy. By combining the bells with a pegging needle telegraph, the lack was made good, and a complete system of indoor signals was achieved. In order to handle greater traffic without delays, the blocks were made shorter than the distance between stations. For example, between Wokingham and Reading on the SER, where there was no intermediate station, trains required about 15 minutes. When an intermediate signal was provided, the maximum wait was reduced to 7.5 minutes.

On 20 July 1852, Edward Tyer (1832-1912) patented a block instrument that combined bells with a visual reminder, illustrated at the right. There were two indicators in the upper part, one above the other and differently coloured, red and black, and referring to the two lines of a double-track railway. These were controlled by the ingenious arrangment shown in the figure below. The soft-iron needle connected with the rotating armature adhered to the permanent magnet on one side or the other. When to the left, as shown, "train on line" was indicated. When to the right, "line clear" was indicated. The indicator could only be in one position or the other. When a current was passed through the coil, the end of the soft-iron needle could be made either a north pole, or a south pole, depending on the direction of the current. If, in the position shown, the current made the end of the needle a south pole, nothing happened. If the current was in the reverse direction, the resulting north pole was repelled by the permanent north pole and the needle moved to the other position and stuck there. Connections were made to the terminals on the sides of the case: L, line; E, earth; +, -, C to the bipolar battery supply.

The upper needle was controlled by currents entering by the line, the lower needle by currents originating in the instrument. Below the indicators were two buttons, or "pistons", for sending signals, side by side and also coloured red and black. Pressing these buttons sent a current of one polarity or the other through the coil of the lower indicator and the line, controlling the lower needle at this instrument, and the upper needle at the instrument in the rear. Incoming signals rang a bell or gong on the instrument. A bell was used for one line, a gong for the other. The bell code was: acknowledgement, 1; passenger train, 2: goods train, 3; express or light engine, 4; obstruction on line, 5; testing, 6. An intermediate block post had two instruments, so that communication was established with the block posts on both sides. Only one wire was necessary for each instrument, since a ground return was used. This wire could be used for bell codes as well as for block information, since the currents were momentary. When sending a bell code, the button under the current position of the lower needle had to be used, to avoid changing its setting.

These instruments seem to have been first installed on 11 miles of the North Kent line of the SER. Since they required few wires, and were easy on batteries because of the momentary currents, they became very popular. They were imported in France, where they were used widely on the PLM and the Ouest. The first was installed at the tunnel of Blaisy-Bas in 1855. Paris-Melun and the tunnel of Saint-Irenée at Lyon followed in 1867. By 1881, 1862 km of the PLM was protected by Tyer instruments, and in 1890 2080 km, including the complete main line from Paris to Menton. The Tyer instruments even had the distinction of being modified by Jules Regnault of the Chemins de Fer de l'Ouest in 1858 to suit French conditions better. In France, Tyer instruments were often used with Jousselin train-describing bells. English observers, however, considered the original Tyer instruments superior to the Regnault version. Regnault seems to have worked with block telegraphs since 1847, but the modified Tyer instruments were first regularly installed after 1887. Instruments operating on momentary currents are always susceptible to derangement by foreign currents. However, Tyer's use of soft iron for the operating needles eliminated the danger of reverse magnetization. In fact, the use of such needles, called "induced" needles, later became common in all needle telegraphs, also greatly reducing the need for continual remagnetization, since the magnetization was provided by sturdy permanent magnets, not delicate needles. Edward Tyer should not be confused with Captain Tyler, R.E., the Board of Trade inspecting officer who wrote the report on the Clayton tunnel accident.

The Tyer block had no interlocking between block instruments and outdoor signals. In the PLM Block No. 1, it was arranged that the block signal could not be cleared when the instrument showed that the block was occupied. The PLM Block No. 2 included further locking, but was still mostly the Tyer block. Finally the PLM Block No. 3 was developed, which replaced all the Tyer blocks, including the PLM Blocks No. 1 and 2, in 1899-1901. PLM Block No. 3 survived until around 1950. This was a lock-and-block system (see below), and the second most used block in France after the Lartigue. See the article on the Lartigue electro-semaphore for an explanation of this system.

The Tyer instruments were operated in the following way (as stated in PLM instructions): when a train left post A, the signalman pressed the button under the current position of the needle for the corresponding line on the instrument connecting him with the station towards which the train was proceeding, after placing his signal at Stop. This gave one sound of the bell or gong at post B. The signalman at B then pressed the "train on line" button on his instrument, which caused the needles at both ends to assume this position. When the train left the block, the signalman at B pressed "line clear" and the needles went to that position. Now the signalman at A could clear his signal and await the next train.

Another method of operation was established by Edwin Clark of the L&NW, who used needle telegraphs, one for each line, as well as a speaking telegraph for communication between adjacent block posts. These were the ordinary single and double needle instruments then available, with new faces and pegs to hold the handle one way or the other. Originally, the block telegraphs were maintained pegged at "line clear" or "train on line", depending on block occupancy. A loop of the telegraph wires was brought down every second pole, where it was easily accessible. In case of trouble, a guard could cut the wire, and the opening of the circuit would indicate danger, as the needle went vertical. This was much more trouble than it was worth, and was soon discontinued. The block system was initially installed in 1855 between London and Rugby, 83 miles, with an average block length of 2-1/2 miles. The short blocks would permit a headway of about 5 minutes for goods trains, and even less for passenger trains, so a heavier traffic could be handled than under the time interval system, something that surprised many managers.

Although an absolute block was intended at the start, in which no train was admitted to an occupied block, it was found necessary to adopt a permissive block in which a train could be admitted to an occupied block after being stopped and warned by the signalman at the entrance to the block. In this case, it was necessary to add one train for each train admitted, and subtract one for each leaving the block, to know when "line clear" could again be given. Clark's block telegraph, operated by continuous currents, was the model for most later British block indicators. It was inherently a three-position indicator, since the needles hung vertical if no current was applied. He apparently used porous-cup Daniell cells, which are well-adapted to closed circuit work. The date seems a little early for the Callaud gravity cell that was widely used for closed circuit work in the United States, which demanded closed circuit operation to keep the solutions separate.

In 1855, the GWR appointed C. E. Spagnoletti (1832-1915) Telegraph Superintendent, who served in this post until 1892. He is best known for the invention of the disc block instrument, which showed a white or red plaque in a window instead of using a needle indicator. The difference with the needle instruments is analogous to the difference between the surveyor's and the nautical compasses, the one reading the position of the needle on a stationary card, the other rotating the card with respect to a fixed index. The first were installed on the Metropolitan Railway on its opening in January 1863. The instrument had two keys that could be pegged down, the white one on the left giving line clear, the red one on the left train on line. When de-energized, half of each plaque showed in the window. The block telegraph between Paddington and Bristol was complete in 1873. The GWR applied block telegraphs first to single lines, only later to double lines. The telegraph with double-needle instruments was general after 1852, and single-nedles after 1860, but was often not primarily used for train operation.

W. H. Preece (1834-1913) invented a block instrument in 1863 that used a miniature semaphore instead of a needle or Spagnoletti's movable plaques. This was an actual miniature semaphore, driven by an electromagnet, rack and pinion. Unknown to him, Tyer had exhibited a semaphore block instrument in 1855, but it was never put into practice. Many block instruments, especially those showing only two indications, were given the form of a semaphore, where the arm was moved by a needle as in an ordinary telegraph. These instruments reinforced the idea of the block instrument as a remote distant signal. The job of the signalman was to make his outdoor signal reproduce the aspect shown on the block instrument, which was controlled by the signalman in advance. One of the first, if not the first, application of the Preece minature semaphore was on the steep gradient between Queen Street (later Central) and St. David's stations in Exeter.

In the United States, block indicators were not used (except very rarely) and reliance was placed on the written block record. Block messages were sent with the ordinary sounder telegraph. The system was, in general, normal-clear. Only on single track was the operator at the other end of the block asked to prevent any opposing trains from entering the block before a train was admitted. It must always be realized that the block system was supplementary to time table and train order, and was not the primary means of authorizing a train to proceed.

There are three elements in a block system: (1) the indoor signals used by the signalmen for communication with one another; (2) the outdoor signals used by the signalmen for communicating with the trains; and (3) the trains themselves. In the basic block system, these elements are independent of each other and must be coordinated by the actions of the men involved.

Let us assume that we have three block posts A, B and C in the direction of the current of traffic. Each block post is equipped with a signal serving jointly as home and starting signal. Let the block system be normal-stop, so that the signals are at stop unless cleared to allow a train to pass. At least a telegraph line is necessary for communication between block posts. Suppose a train arrives at A and wishes to proceed. A gets the attention of signalman B and asks: "Is line clear?" This is called offering the train. Signalman B checks his block book, and if the line is indeed clear replies "Yes" in some way. Then signalman A may clear his signal and allow the train to enter the block. He replaces his signal to stop after the train, and notifies B "train on." B then acts similarly with respect to post C, offering the train and receiving acceptance. He clears his signal and allows the train to pass. When it does so, he replaces his signal and notifies A "train out."

All this can be done with a speaking (alphabetical) telegraph, but it is very convenient to use a single-stroke bell for the messages, and a block instrument as a visual reminder of the status of the block. Both of these are telegraphs, fundamentally, but special ones. Now a train can be offered by A with the bell, and accepted by B by repeating the bell code and turning the block indicator from "normal" to "line clear", which appears on A's instrument as well. When A sends "train on" with the bell, B replies by turning the block indicator to "train on" and repeating the bell code. When the train leaves the block, B merely needs to turn the block indicator to "normal." If desired, a bell signal can be sent additionally, but it is not really necessary.

Block indicators may be two- or three-position. Those descended from the needle telegraph are three-position, since the current can be of either polarity or absent. Those descended from Preece's miniature semaphore are two-position. Usually, "normal" is represented by one or the other of the semaphore positions, usually the arm horizontal, on a two-position instrument. The two-position indicator shows the proper position of the outdoor signal. Upon the adoption of the three-position block, one of the two indications of a two-position block instrument had to be chosen to represent "Line Normal". Since the Train on Line position was generally chosen for this, it now meant: (1) Train on Line, and (2) Train Not on Line. It would have been more logical to have chosen "line clear", but the problem with this is obvious. I have seen three-position instruments used as two-position instruments very recently. On giving "train out", the signalman turns the block indicator to "line clear" instead of "line normal". When the next train is offered, the signalman only repeats this, since the block indicator is already at "line clear". Train register entries do not betray this, since the time of "is line clear?" does not have to be recorded when acceptance is immediate.

In Britain, beginning in the late 1850's, levers for operating signals and points were concentrated in one location, and mechanical means provided for ensuring that they could not be operated in an unsafe manner. This interlocking was achieved by interfering directly with the motion of a lever, or with the movement of the catch handle which was necessary to move a lever. It is also possible to provide levers with electrical locks, operating either on the catch handle or the lever. When de-energized, these locks prevent moving the lever. When energized, the lever can be moved. To save power, the locks are not usually energized until a button is pressed above the lever that establishes the electrical connection. Electric locks provide a means of further interlocking of the levers, and of coordinating the levers with the block system. A system combining mechanical interlocking and electrical interlocking came to be known as "lock and block", or "controlled manual block".

Two trains can be admitted to a block erroneously in the following ways: first, a signalman may offer a second train before the first one has left the block. This is not an error in itself, but should the signalman in advance accept the train, a dangerous situation is created. He may do so if he has been distracted, or sleeping, and is uncertain whether the first train has left the block and has escaped his notice. If sufficient time has elapsed, he may think it has, and try to cover his inattention by accepting the second train and fudging the entries in the block book. If his signals are at clear, this belief may be reinforced. Secondly, a signalman may not have replaced the signal at stop behind a train, and accepts a second train. The second train then accepts the signal intended for the first train and enters the block behind it. These errors are very rare, but they have occurred.

Proper procedure may be enforced electrically by locking the signal levers. Signalman A may not be able to operate his signal until it has been unlocked by signalman B as part of the reply to "Is line clear?" Signalman B, in turn, may not be able to do this unless he has put his own signal at Stop after a preceding train. Also, B may not be allowed to unlock A twice in a row without having been unlocked by C. All of this ensures that trains have probably progressed from A to C. To ensure that trains have actually physically passed requires the intervention of the train in addition.

Let the track layout be as in the diagram. Just in advance of each signal is a treadle operated by the wheels of the train. When B unlocks A's signal, A can clear the signal for a train, but this action again locks the signal. In this condition, with the signal cleared, it should be impossible for A to accept another train. When the train operates the treadle, the signal is again unlocked, and it can be replaced at stop, where it is again locked. B cannot give A a second unlocking until the train has passed B's signal, operated the treadle, and the signal has been replaced. In this way, the progress of the train is guaranteed, and neither of the errors mentioned in the preceding paragraph can be committed.

A system like this was invented by W. R. Sykes in 1874, patented the next year, and widely applied after 1878 in Britain and elsewhere. A typical two-position instrument is shown at the left. It combines a semaphore block indicator, an upper tablet showing received signals, and a lower table showing signals sent. Signals are sent by means of the plunger, which is pressed in, and the switch hook, which can be swung to rest on the plunger rod and prevent its operation. This instrument is used in connection with the usual bells.

The instrument is shown in the normal state. To offer a train, A uses his bell as usual. If B can accept the train, he presses the plunger which sends a signal unlocking A's instrument, so that A's upper tablet now shows "free", and his signal lever can be operated. When A sends "train entering section", B moves the switch hook over onto the plunger, which changes the block indicator to "train on" and also reminds him not to press the plunger again. The lower tablet of A's instrument now shows "train on". This is additional to the block indicator, but some instruments do not have a block indicator and the lower tablet provides useful information. A permits the train to proceed, after which his signal is again at Stop and the upper tablet states "locked". When the train passes B, the signal there is restored to Stop (after it is freed by the treadle), and the switch hook is moved off of the plunger. This changes the lower tablet to blank at A and B. B may now accept a following train from A.

If a train that has been accepted cannot proceed, a key is rotated in the release keyhole that clears the instrument. The use of the key is noted in the block book.

Some instruments do not have all the parts shown. A receiving-only instrument with no block indicator, plunger or switch hook, only a single tablet showing Locked and Free, may appear on the block shelf above a signal lever. A transmitting instrument will have a plunger, switch hook and release keyhole, but only one tablet and no block indicator. A double-arm block indicator may be in a separate case, with a bell tapper and a communtator for operating it. Points may be locked as well as signals, to protect a route on which a train will be accepted. Trains may not be accepted independently on two routes approaching a junction. The electrical interlocking can be quite extensive, supplementing the mechanical interlocking and integrating block functions.

The Sykes was installed in 1892 on the New York Central from New York to Buffalo, 440 miles, and about the same time also on the New York, New Haven and Hartford from New York to Providence, 176 miles, and on the Erie from Jersey City to Turners, 46 miles. Some of these installations were in service before 1890. The Sykes was first used in the Fourth Avenue tunnel in New York City before 1888. Between Providence and Boston, 44 miles, the New Haven used J. P. Coleman's new instruments from Union Switch and Signal. Coleman's instrument had a semaphore block indicator and a single tablet showing Free, Train in Block and Locked. It used track circuits instead of treadles, and electric slots to operate signals. An electric slot is a mechanism in the up-and-down rod that maintains the rod connected when energized, but disconnects the rod when de-energized. In effect, it gives dual control of the signal. It was used to return a signal to Stop after a train shunted the track circuit, independently of the signalman. It was a general tendency in lock-and-block to replace mechanical actions by electrical ones as the systems were redesigned and improved with time.

Modern electrical block controls used in Britain provided most of the functions of the Sykes in a simpler and less obtrusive way, eliminating the plunging and other special operations. Short track circuits were provided in rear of the home and starting signals to prove the presence of trains. For example, the starter could not be pulled off unless the block indicator was at "line clear", and when replaced could not be cleared again unless a new "line clear" was received, or perhaps until the train had occupied and exited a track circuit at the post in advance. All of this is rendered unnecessary with continuous track circuits and semi-automatic signals (controlled jointly by the signalman and by the track circuits). In the United States, controlled manual block was originally used to avoid long track circuits. After the application of continuous track circuits, it disappeared.

At about the same time as Sykes's invention of his block instrument, Siemens and Halske developed a block machine working on a completely different principle. The external appearance of a machine for an intermediate station is shown at the right. M and M' are cranks for operating semaphore signals, one arm for each direction. The handle h rotates a magneto, providing an alternating current for operating the machine. Coloured screens appearing in the two circular windows show the states of the blocks in the two directions, as indicated by the arrows below them. If a white screen appears, the semaphore can be moved to clear or stop as desired. If a red screen appears, the semaphore is locked at stop. To cause a red screen to appear, the signalman rotates the handle h while pressing on the knob of the screen to be made red. This generates an alternating current that works an escapement both at this office and the one in the rear. The one in rear, initially red, becomes white at the same time due to its own weight. A signalman may cause a screen to go red at his station, but cannot clear a red screen that has appeared. This can only be done by the signalman in advance. Therefore, the signalman in advance in effect clears the signal in the rear at the proper time, when he is protecting the train at his station.

The interior of the block machine is shown at the left. The left-hand aperture shows a red screen, the right-hand one a white screen. Note that the crank operating the signal on the right can be rotated as desired, but the left-hand one is locked at Stop. The wiring is not shown, but the magneto is in the centre, and there is a pair of electromagnets on each escapement. One end of the escapement rod rings a vibrating bell that shows when current is being received. Other bells, not shown, are available for communication. The covers o and o' (in the preceding figure) can be removed so that the escapements can be operated by hand in an emergency to make the proper colour appear. The use of magnetos instead of batteries is typical of Siemens and Halske, and this feature was greatly liked.

Siemens and Halske block machines were widely used in Germany and Holland, and were tried in Belgium and France. Like the Sykes, they were continually improved. They were never used in Britain or America, since they do not fit well with the operating practices of either. A block system with a similar nature, but completely different construction and operation, was used in France, the electro-semaphore of Tesse, Lartigue and Prud'homme, invented in 1872. In this system, the outdoor and indoor signals were the same, so they could never disagree. See the article on this semaphore for more information. The principal aim of these block systems was to integrate the outdoor and indoor signals so that they could not conflict. This was never an issue in Britain and the United States.

After this discussion, it may be useful to give a classification of block systems. This outline was given by Flamache in his report to the 4th International Railway Congress (translation).

1. Indoor and outdoor signals are independent.

2. Interlocking between indoor and outdoor signals. Satisfies (a) and sometimes (b) and (c).
(a) When a signal is put to stop, it cannot be cleared again until the post in advance has returned its signal to stop.
(b) The signal at the post in rear cannot be cleared until the post in advance has returned its signal to stop.
(c) Two unlockings to the post in rear must be separated by an unlocking from the post in advance.

3. Independent means of assuring occupancy by one train.
train staff, staff-and-ticket, electric train staff, electric tablet

4. Action of trains cooperates with actions of signalmen.
(a) Outdoor signals operated manually.
(b) Locking of a signal put to stop until released by post in advance.
(c) Possibility of multiple unlockings to post in rear when signal is at stop; inability to send the first such unlocking until signal has been put at stop.
(d) Absolute necessity of separating two unlockings to the post in rear by an action by the train exiting the section in rear.

5. Automatic operation by trains.

L. T. C. Rolt, Red for Danger, 3rd. ed. (Newton Abbot: David & Charles, 1976). Clayton tunnel is discussed on pp. 51-57.

W. H. Preece, On Railway Telegraphs, and the Application of Electricity to the Signalling and Working of Trains, Proc. Inst. C. E., 13 January 1863, pp. 167-239. This is essential reading for the early history of railway telegraphs.

W. F. Cooke, Telegraphic Railways (London: 1842). A copy is in the Science Museum Library, London.

E. T. MacDermot, History of the Great Western Railway, Vol. I (London: Ian Allan, 1964). Reprint of 1927 edition. The Box tunnel telegraph instructions will be found on p. 320f.

G. Pryer, A Pictorial Record of Southern Signals (Oxford: Oxford Publishing Co., 1977). Sykes instruments: pp. 151-157.

G. M. Kichenside and Alan Williams, British Railway Signalling (London: Ian Allan, 1963). Sykes: pp. 43-46.

M. Christensen and D. Stirling, Symbols for Sykes Drawings, The Signalling Record, No. 103 (2004), pp. 2-11.

Sartiaux, Note sur le Block-System et sur quelques Appareils, Annales des Ponts et Chausées, Mémoires, Tome XIV, pp. 329-369 (1877).

Flamache, Question XVI: Signaux Fixes et Block-System, 4th International Railway Congress, Mémoires et Documents, pp. 929-935 (1900).

M. Bouvet, La Circulation des Trains sur un Grand Réseau Ferré, La Technique Moderne, 16, pp. 597-603, 633-644 (1924).

B. B. Adams, The Block System of Signaling on American Railroads (New York: Railroad Gazette, 1901).

Composed by J. B. Calvert
Created 22 July 2004
Last revised 19 December 2005