The Lartigue Electro-Semaphore


  1. Introduction
  2. Normal-Clear Block Working
  3. The Electro-Semaphore
  4. The Palette SEM
  5. References


The telegraphic block system for railway operation was suggested in a pamphlet by W. F. Cooke in 1842, and put into practice in 1844. In this system, the line is divided into intervals called blocks, with a signalman at each block post at the points of division. When a train arrives at a block limit, it is allowed to proceed if the next block is unoccupied, and a fixed signal is displayed to protect its rear. The signalman at the exit of the block is notified of the entry of the train into the block. When it arrives complete (as shown by markers on the last car), he notifies the signalman at the entry to the block by telegraph that the block is now clear, and this signalman can clear his signal ready for a following train. For simplicity, we shall consider only double-track railways where the current of traffic on each track is constant.

On single track, where trains may move in either direction, it is necessary for the signalman at the entrance to the block to obtain the cooperation of the signalman at the exit, to ensure that no train is admitted at that end, before allowing a train to enter the block. Also, if a train on double track finds the block in front of it occupied, it may be allowable for the train to proceed on the understanding that it may overtake a train in the block, and therefore must proceed cautiously so that it can stop well within the distance that can be seen (French, marche à vue). This is called permissive working, and is applied for trains other than passenger trains. If only one train is allowed in the block under any conditions, the block is called absolute.

There are many details to practical block working, such as the fixed signals used, the means of communication, and the keeping of block records. In Britain, signalmen occupied signal boxes, controlled semaphore signals remotely with levers and wires, communicated by codes of strokes on single-stroke bells, as well as with a block instrument for each track that constantly showed the state of the block (occupied, free and not in use), and kept written records of all bell signals and their times. The block system soon became the primary means of train operation, drivers proceeding on signal indications alone, and after 1889 it was required by law on all passenger lines. By that time, the block system was already widely used, and the law simply confirmed standard practice. Railway managers found to their surprise that track capacity was increased by the block system, not decreased as they had feared. In America, telegraph operators, who handled train orders issued by the train dispatcher, also managed the block system from their offices, used the train-order signal to block trains, communicated by Morse telegraph (in most cases; sometimes bells were used), and kept a record of the times signals were sent and the state of the block. The block system was always supplementary to the timetable and train order movement of trains in America, an added safeguard, not a primary method of operation.

The British block instrument was basically a needle telegraph operated by constant currents, giving a visible indication of the state of the block. It was operated by the signalman at the exit of a block, and was displayed both there and at the entrance to the block. With no current, the needle was vertical and indicated "line blocked," which meant only that the line was not in use. When the signalman (A) at the block entrance wanted to send a train, he asked permission of the signalman at the block exit (B) by a bell code. If the block record showed the block clear, B turned the block instrument to "line clear," an action called "accepting a train." Seeing this, A could then clear his signals to admit the train to the block. When the train passed, A restored his signals to stop and notified B by bell that the train had entered the block. B then set the block instrument to "train on line," which was duly noted by A. When the train left the block, B notified A by bell, and restored the block instrument to "line blocked." The times of all bell signals were recorded at each block post. Later, electrical interlocks were provided to ensure that the signals were operated in accordance with the block status. The block instruments and bells are called indoor signals, the semaphores observed by drivers are called outdoor signals. The signalman was the connection between the two.

Electrical checks on proper operation are made by electrical locks on the levers used to operate signals and switches. An example of an electrical check on the block system is arranging that when "line clear" is given on the block instrument, the home signal can only be cleared once. This prevents admitting a second train in error without securing the cooperation of the signalman at the block exit. The greatest danger in the block system was never in its improper operation, but simply forgetting that a train had been shunted to the other track and signalling a train into it.

In France, trains were operated on the principle that a train stopped unexpectedly or moving at a walking pace on the main line was protected (couvert) by the conducteur d'arrière, who placed torpedoes (pétards) on the rail at intervals of 1 km. To allow time for the protection of trains, a time interval of 10 minutes was enforced as well as possible by every employee along the line. This actually works better than might be imagined, was used in Britain in early days, and always in America ("Rule 99"), but now and then it can fail badly. The necessary procedures for it also take additional time, so the line capacity is severely reduced. Clearly, six trains an hour in each direction on a double-track line is the upper limit of capacity. This system was always used in America (where six trains an hour was very dense traffic indeed) to protect the rear of trains by flagmen in the absence of other signals, but is satisfactory only for light traffic and moderate speeds. It is usually called the time-interval system, but this term is inadequate.

The French term le block-system clearly demonstrates its British origin, which occurred in the 1850's. By 1875, about 50% of British lines were under the block system, including all those of heavy traffic, and many isolated blocks protected tunnels and other points of special danger. The block system became obligatory in Germany from 29 December 1871. In 1873, Belgian railways were required to use the block system, and in the Netherlands from 27 October 1875. Tyer's block instruments from Britain had been used for some time on 15 km of the Ceinture de Paris line, as well as on some lines of the Ouest, the Est and the P-L-M. Regnault's apparatus, a modification of Tyer's, was used on the busy Versailles line. This article discusses the electro-semaphores of Lartigue and Tesse, patented in 1872, and first used on the 44 km Saint-Denis to Creil line via Chantilly of the Chemins de Fer du Nord in October 1875. These semaphores spread throughout the Nord, and into Belgium, and were still in use a hundred years later.

In the British and American block systems, the signalman is the connection between the indoor and outdoor signals, operating the outdoor signals seen by the driver in accordance with the indoor signals connecting him with the neighboring block posts. The Lartigue electro-semaphore represents at one time both indoor and outdoor signals, and so they cannot be operated out of correspondence. Electrical methods of coordinating indoor and outdoor signals were called lock and block, or, in America, controlled manual block. These systems generally used track circuits or treadles to detect the presence of trains.

It is customary in American and British signalling practice to say that a train coming up to a signal is in rear of the signal, but after it passes will then be in advance of the signal; that is, to take the point of view of the train, not the signal. In common language, however, as well as in some of the references, the use of "in advance" may imply exactly the opposite. In French, the terms are d'amont for the direction in which a train is coming, and d'aval for the direction in which it is moving; the analogy is with a current of water. Trains are correspondingly said to be montants or descendants.

An essential part of many electrical signalling systems from the late 1800's was the Hughes electromagnet. It consisted of a strong permanent horseshoe magnet with soft iron pole pieces, around which coils were wound. With no current in the coils, the armature is strongly attracted. With one direction of the current in the coils, the resulting magnetism is opposed to that of the permanent magnet, weakening the attraction so that the armature can be pulled away easily by a spring or gravity. If the current is in the reverse direction, the magnetic flux is increased, and the armature is more strongly attracted. Therefore, the Hughes electromagnet responds only to currents of one direction. Its advantage over a normal polar relay is that it is considerably more powerful, and can do mechanical work, such as unlocking the large or small arms of the electro-semaphore, which then move under the influence of gravity. The Hughes holds an arm against gravity; when it is energized by a current of the proper direction, the operating rod is released. The Hughes was also at the core of the cab signals operated by the crocodile. Since the currents used are intermittent, the Leclanché cell was a satisfactory power supply.

Normal-Clear Block Working

To understand clearly how the Lartigue electro-semaphore block functions, it may be helpful to review the normal-clear block system and two-position block instruments, which were actually used for many years in Britain. The Tyer block instrument appeared as a miniature semaphore, but was fundamentally a needle telegraph. When current flowed through it, the arm of the semaphore was depressed, indicating "Line Clear." When the current was interrupted, the arm rose to the horizontal, indicating "Train on Line." These were the two states of the block, and Line Clear was the normal condition.

Let us consider only one block, with trains in only one direction. Suppose signalman A is at the entrance to the block, and signalman B is at its exit. Signalman A controls a starting signal that admits a train to the block. This is, of course, a large outdoor signal. Signalman B controls the block instrument, and signalman A can observe a duplicate in his own office. The signalmen are in communication through a single-stroke bell, which is quite sufficient for the few standard messages they must exchange. Normally, the starting signal is "off" and the block instrument shows Line Clear. If a train arrives at A and enters the block, signalman A protects it by putting the starting signal at Stop, which prevents any following train from entering the block. He then gives B two beats on the bell (after any preliminaries), which signifies "Train entering section," or for short, "train in." B then unpegs the block instrument to show Train On Line, which is duly observed by A. If any following train appears at A, this reminds signalman A that the section is occupied.

Signalman B awaits the train, and when it passes notes that it is complete. When it is clear of the block, he sends signalman A the bell message 2-1, "Train out of section," or "train out" for short, and pegs the block instrument, so that it shows "Line Clear" and reminds signalman A that he may pull off his starter. We have now returned to the original condition. At the present time, things are complicated a little by introducing a third state, "Line Blocked" and leaving the starter "on". Now A must first offer a train to B, and if it is accepted B pegs the block instrument at "Line Clear" and A may clear his starter. There are several advantages to this latter system, but it is really no safer than the first one, just more convenient (every time the clearance space at B is infringed, it is not necessary to block back and tell A to put his starter at Stop).

In the Lartigue system, signalman A has an outdoor signal that he displays to protect a train entering the block. This mechanical action causes a current to be sent to signalman B that releases a small semaphore arm there to inform B that a train is coming. This is the "train in" signal. The large arm at A takes the place of the arm of the block instrument; it is, in effect, a large outdoor block instrument. When the train clears at B, signalman B mechanically cranks the small arm back to normal, which sends an electrical signal to A that unlatches the large signal and allows it to fall to Clear. This is just the "train out" signal. This operational procedure should be kept in mind when reading about how the system operates. It is, incidentally, much cheaper than an equivalent British block system and uses much less equipment.

The Electro-Semaphore

Henry Lartigue (1830-1884) was born on 30 September 1830 in the village of Saint-Mandé to Bernard Lartigue and his wife, Auguste Luchsenring. Educated in provincial schools, he went to the Observatory of Paris, then under Le Verrier, to help organize the meteorological service, in 1854. Lartigue was fascinated by meteorology and electricity. Le Verrier's direction proved odious, so Henry resigned with other malcontents, and joined the Chemins de Fer du Nord in 1856 as a trainee. He was soon named Controller of the telegraph service at Paris, where his supervisor was Paul Tesse, just named Special Controller. He married Emilie Lasserre in 1862, and had two children, Bernard and Caroline, born in 1863 and 1865. He worked with the Digney brothers and Paul Prudhomme, Paris industrialists, and his associates at the Nord, Louis Forest and Paul Tesse. On 16 January 1872, Lartigue, Forest and the Digney brothers patented an electropneumatic valve that became the crocodile. On 24 February 1872, Lartigue, Tesse and Prudhomme patented the electrosemaphore. Lartigue made many other inventions, and won many gold medals. He died of an epidemic in Paris on 17 November 1884.

A Lartigue electro-semaphore is illustrated at the left, as seen by an approaching train. Generally, one mast was equipped with signals for both directions, but only one set is shown here. The signal is displaying the aspect "Stop, block occupied." Note that the night aspect shows one red light and one green light, produced by one lamp and a reflector. Two red lights was the night aspect of the carré, which could, therefore, be distinguished from the semaphore. Both aspects commanded an absolute stop before passing the signal. Leclanché batteries were located in the box at the bottom of the mast.

The large arm, 2 m long and 6-12 m above the ground, is weighted so that it hangs vertically along the mast when released. This arm is painted red on the front and grey on the back. It is brought to the horizontal position by rotating the handle of the operating box at the left by a little more than half a turn, until a catch is engaged. When electrically released by the post in advance, a Hughes electromagnet in the operating box releases the catch and the arm goes to vertical. An arm on the operating rod rings the bell on the mast.

The small arm, 90 cm long and 4 m above the ground, is for the information of the signalman, the agent de poste. It is painted yellow (Nord, Est) or white (P-O) on the front and gray on the back. It is weighted to assume a horizontal position when released. It is brought to the vertical position by rotating the handle of the operating box on the right by a little more than half a turn. In the diagrams it is shown as red, since yellow and white would not show up well. The large and small arms for the same direction are on opposite sides of the mast, above the corresponding box 1 and box 2, though shown on the same side in the diagrams for clarity.

Line wires extend to the post in advance and the post in rear. When the post in advance brings his small arm vertical, a signal is sent that releases the large arm at this post. This is a signal that the block is clear. When the post in rear brings his large arm horizontal, a signal is sent that releases the small arm at this post. This is a signal that a train is approaching in the block. Trembling bell signals can also be sent to the posts in advance and in rear for communication purposes. We will now describe the method of signalling a train in detail.

The figure shows the Lartigue electro-semaphore in a resting state, when the block it protects is not occupied, and no approaching train has been announced. The small arm is shown in red so that it will be more visible in the figure. When a train enters the block at the post in rear, the signalman at that point raises his large arm to horizontal to protect it. This action also sends a signal to the present post that releases its small arm, announcing that a train is in the block. Imagine that this has not yet occurred, and the small arm is vertical, where it has been set by rotating the handle of the electromechanical box on the right (No. 2) at the foot of the mast after the passage of the preceding train. When this was done, a signal was sent to the post in the rear that unlatched its large arm, by means of a Hughes electromagnet, allowing it to fall to a vertical position. Since the crank has been rotated past the 180° position, the released crank now completes its rotation under the weight of the arm. The signalman must operate the small arm only when he has seen the markers on the train that is leaving the block in the rear, so he is sure that the block is clear. A small window at the top of each electromechanical box showed "libre" or "occupé" depending on whether it was released or not. This was done by means of a second Hughes electromagnet operating on a polarity opposite to that of the main electromagnet. The electric signals sent by the operating box are of one polarity when the arm is being raised, of the other when the crank completes its turn, due to a commutator rotating with the crank.

When the signalman at the post in rear protects a train by raising his large arm, a pulse is sent to this post that releases the small arm and allows it to assume a horizontal position. This small arm can be imagined as repeating the position of the large arm in rear at the present block post. The bell on the mast is rung mechanically when the rod to the small arm is electrically released, to warn the signalman that a release has occurred. The signalman looks out for the approaching train, and when it has passed his distant signal (a disque rouge), puts the wire-operated distant signal to "on" to protect the train. As soon as the train has passed his signal, he then operates the left-hand electromechanical box to raise the signal to stop. This automatically sends a signal to the post in advance, warning that a train is on line, where the bell rings and the small arm goes horizontal.

Conditions are now as in the diagram on the right. Note that the distant signal is already displayed to protect the home signal. A soon as the markers are seen, the signalman rotates the right-hand handle, which moves his small arm to vertical and at the same time sends a signal to the post in the rear that the train has left the section, which releases its large arm to assume the vertical position and rings the bell.

The signal now appears as at the left, the horizontal large arm protecting the block. When the train exits the block, the large arm is released when the signalman in advance brings his small arm vertical. This also rings the bell to attract the attention of the signalman at our signal. Now the signal is again as shown in the first diagram, with both arms vertical.

The line wires to the posts in advance and in rear each handle two kinds of signals, distinguished by their polarities. A signal from this post unlatches a signal arm at either of the two posts, using a Hughes electromagnet. A signal arriving on either line rings the bell and unlatches one arm or the other. By the use of polarity, only one line wire is needed for each track. The system is powered by 12 Leclanché cells (18 V) in the box at the foot of the mast. The lantern for showing the night aspects is not shown, but it was behind the circular opening in the arm, which carred a red glass. The lamp could be raised and lowered for maintenance. When the arm is horizontal, the lamp shines through the roundel, which contains a red lens. When the arm is vertical, a white light was shown. A mirror may be used so that two lights are shown, one on each side of the mast. These were red and green, the green on the right produced by reflection. After the introduction of the Signalisation Verlant in 1934, a spectacle with red and green lenses was used instead.

The actual arrangement of the control boxes is shown in the figure at the right. As you face the signal from either side, a box 1 is on the right, a box 2 on the left, and they both refer to the next station in the direction you are facing. The two boxes back-to-back on each side pertain to a track. The electrical command transmitted by rotating the handle of a box 1 is equivalent to "train on line", which releases the small arm at the signal in advance. This is denoted by "a" on the figure. An incoming "a" signal is shown by an arrow on the line to box 2, where it releases the small arm. Box 2 transmits a signal equivalent to "line clear", denoted by r, to the signal in rear. This signal is shown being received at box 1 to release the main arm. Note that "a" is sent in advance, "r" to the rear, for each track independently.

It can be seen from this discussion that the block system can be worked by les agents de poste with no further assistance. For safety, it is necessary only (1) to raise the home signal to horizontal to protect any train that has passed it; and (2) to raise the small arm to vertical only after the markers of the train leaving the block are seen. The simplicity of the system makes it very robust. Failure of the batteries or of the line wires cannot result in danger. A signalman cannot clear his own signal; this can be done only by the post in advance. No special training is required, unlike in the case of the long apprenticeships and special skills of the British signalman or the American telegrapher. The Lartigue electro-semaphore was, essentially, a large outdoor block instrument operated by intermittent currents, and obeyed directly by the drivers. The signalman's part was to supply the energy to move the semaphore arms at the proper times. Nothing like it was ever used in Britain or America.

The most serious malfunction was the accidental release of the large arm, for example by earth currents caused by thunderstorms. This caused a certain amount of worry, but failures were very few.

In the case of permissive block, the trains are admitted after warning past the home signal at stop. When the post in advance clears the home signal, the signalman must at once raise his signal again, and repeat this as many times as additional trains have been admitted to the block.

In the basic system, there is no interlocking between placing the main arm at stop (by rotating the handle of box 1) and sending line clear to the station in rear (by rotating the handle of box 2) while winding the small arm vertical. The nature of the apparatus would render it very unlikely that anyone would leave the main arm at clear while restoring the small arm, contrary to the habit of first protecting the train that has just passed. Nonetheless, the Nord put a small interlocking box for this purpose between the two boxes for each direction, and similar interlocking was made general.

Beginning in the 1970's, the main arms were replaced by light signals placed at the departure ends of the station, and the mast was cut off above the level of the small arms. In this form, the Lartigue block survived into the 21st century, but in continuously diminishing mileage. The last two-armed semaphores disappeared in March 1998.

The Palette SEM

The Nord found it desirable to provide a distant signal for the Lartigue semaphore. This was also a semaphore, called the palette SEM, and generally replaced disques rouges. In 1923, the Nord installed 214 palettes SEM between Creil and Calais, 250 km. These were wire-operated, like the discs they replaced. The sheet metal arms, 6 or 7 m high, were painted green, with a black and white border and white transverse stripes. The curious thing about them was the night aspect, which consisted of lights reflected along the arm by mirrors from a lamp at the pivot. When the arm was horizontal, the lights were green, while when the arm was inclined at 45° the lights were white, giving the appearance of a luminous band.

The Nord installed palettes SEM generally on its lines. The original appearance is shown in the figure. When the Verlant reforms came along in 1934, it did not wish to change to the yellow squares rotated 45°, so the arms were painted yellow, and displayed yellow or green lights by night. The Nord-Belge, which used the Lartigue semaphore, also adopted the palette SEM.


M. F. Clerault, Note sur l'emploi des électro-sémaphores de MM. Lartigue, Tesse et Prudhomme pour la réalisation du block-system, Annales des Ponts et Chausées, Tome XIV, Mémoire No. 49, pp. 197-257, 1877.

M. Sartiaux, Note sur le block-system et sur quelques appareils, Annales des Ponts et Chausées, Tome XIV, Mémoire No. 55, pp. 529-569, 1877.

B. Dieu, Histoire de la Signalisation Ferroviaire en Belgique (Bruxelles: P.F.T.-T.S.P., 2003), pp. 139-150.

G. Dumont, Traité Pratique d'Électicité Appliqué à l'Exploitation des Chemins de Fer (Paris: E. Bernard et Cie, 1885), pp. 265-276.

C. Lecomte, La Vie du Rail, 15 March 1959, pp. 7-10. Biographical sketch of Henry Lartigue.

Brevet d'Invention No. 94,306, Tesse, Lartigue and Prud'homme, 30 Mai 1872, and additions of 1873, 1877 and 1883.

D. Wurmser, Signaux Mécaniques, Tome 1 (Grenoble: Presses et Editions Ferroviaires, 2007). pp. 98-103.

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Composed by J. B. Calvert
Created 30 May 2004
Last revised 16 March 2008