Centralized Traffic Control


[CTC Animation] At the right is an animated sequence showing how a meet between two opposing trains on single track was handled by a dispatcher using Centralized Traffic Control (CTC). The example represents a Union Switch and Signal dispatching machine of 1940's vintage, but many were still in service until very recently. The drawing represents only the essential parts of a typical machine. When you have finished reading this description, you can return here to watch the animation by clicking on Return To Top Of Page. To rerun the animation click on Refresh or Reload. This page is currently under development, and is being improved and refined. Photographs and illustrations will be added as they become available. Different companies could well insist on differences in labeling, colors of indicator lights, and other details. We have chosen what seems to be the most widespread practice, but would like to hear of any peculiarities that existed.

CTC was a registered trade mark of Union Switch and Signal, so some companies called it Train Control System (TCS) instead. However, CTC has passed into the lexicon as a generic term, and will be so used here. Under CTC, trains moved by signal indication, instead of by time table and train order. The dispatcher was constantly aware of the position of each train as it was electrically reported, and controlled the switches and signals remotely. The time required for the transmission and delivery of train orders was saved, as well as that necessary for train crews to open and close switches. For this reason, CTC could give a single-track line a capacity approaching that of a double-track line operated by standard means, estimated at 75 per cent, and so was often installed instead of double-tracking. With CTC, there was no more need for operators or for levermen at interlockings. CTC, in effect, made the railroad one long interlocking.

Each siding switch became a control point, where the signals and the switch machine were interlocked. The switch could not be thrown unless the signals displayed stop, and the signals displayed aspects that depended on the position of the switch and the direction selected. Only two controls were needed for each control point, a switch control, and a direction control. These were called the switch and signal levers, terms from mechanical interlocking. They were rotary switches moved by thumb and forefinger. The switch lever had two positions, normal and reverse, for the closed and open positions of the switch. Indicator lights showed the positions of the switches, and sometimes when the switches were unlocked. A typical switch motor required 7 to 8 seconds to unlock, move the points, and relock. Most switches were dual-control, which meant that they could be released to be operated by an employee on the spot, as well as being remotely operated. The switches were approach-locked so that they could not be changed in the face of an approaching train. A relay house, telephone box, and call light were provided at each control point.

The direction control or signal lever had three positions, those at either side for the two possible directions of movement, labled R and L, E and W, or N and S. A third, center, position that would hold all signals at stop was also provided. Indicator lights showed that the signals had responded to the commands. On some machines, a row of buttons just below determined whether the signals would "stick" at Stop behind a train, or would clear automatically when the track circuit became unoccupied.

The switches and signals were controlled in two different ways, either by direct wire in the vicinity of the office, or by pulse codes sent over a wire to distant locations. When switches were controlled by direct wire, an out-of- correspondence light came on above the switch lever when it was moved and the switch was unlocked. If it failed to go out, something had prevented the switch from completing its travel properly. When the switch had moved and locked properly, the corresponding indicator light came on. Signals operated in the same way. In this case the CTC was equivalent to a small interlocking plant.

When operated remotely by codes, a command code was sent over a wire addressed to one of the 35 possible addresses, which corresponded to control points. If you had more than 35 control points, you needed more wires. When the remote location had executed the command, it returned an indication code to the machine. When one operated a switch or signal lever, the indication light went out, and the codes were stored in the machine. Then, the corresponding button in a row of buttons at the bottom of the board, called code starting buttons, was pressed to send out the command codes stored in the machine. As the commands were carried out, the indication codes returned, and the machine displayed the new condions. Two indicator lights on the panel showed whether a command or an indication code was in process of transmission or reception, and another button canceled stored command codes in case the dispatcher changed his mind. Every time you moved a lever, you stored another command code, of course. A code required 3.75 seconds for transmission. When operated by code, switch levers had only two lights. They went out when the lever was moved, and came on again when the indication was received.

In the simulation, the code starting buttons are not shown; simply assume they are pressed whenever a lever is moved. The animation is considerably speeded up, so that you do not become bored. Trains spent minutes between control points, not seconds, so there was plenty of time to move the controls and to estimate how fast trains would move.

The switch and signal levers were numbered, usually from left to right, with turnouts receiving odd numbers and signals even numbers. These numbers appeared on the index plates for the levers, as well as on the track diagram. Each signal in a numbered group was sublettered according to the position of the signal lever that cleared it. Which signal cleared depended on the position of the switch. The controlled signals were "absolute" signals, in that a train stopped by a signal could not proceed after stopping, but had to remain until further instructions were received.

Between control points were one or more intermediate signals, which operated automatically according to the aspects of the controlled signals, and the presence of trains. In fact, all signals in CTC were semi-automatic, which means that they were dually controlled by the dispatcher and by the presence of trains. In the animation, intermediate signals are not shown, but you can imagine them governing the approach to the aspects displayed by the controlled signals. Most installations had at least one intermediate signal between control points, which could be of a different type than the absolute signals at the control points. Intermediate signals were identified by their mile post location. This number was shown on a number plate on the signal post to indicate that its red aspect was Stop-and-proceed, which was, of course, safe since opposing trains could not get into the block between control points, even if the dispatcher made a mistake, because the interlocking would not allow it. Intermediate signals not only controlled the approach to the signals at the control points, but permitted trains to follow one another closely.

The CTC board, or dispatching machine, was not an interlocking machine, but a control console. It was 54" high and 16" deep, with or without a desk, and came in 2.5 ft and 5.0 ft sections, which were placed side by side as necessary. Inside the cabinet were the switch and indicator lamp bodies, relays, wire cables, power supplies, a bell, and so forth. The single-stroke bell announced the arrival of a train at an OS track section. It could be turned off for most sections so that it only announced the approach of a train to CTC territory. From top to bottom, the board showed a static track diagram with mileages and siding capacities, the illuminated track diagram showing the position of trains, a row of switch levers, a row of signal levers, toggle switches to turn on a light at each control point for calling a maintainer, and the code starting switches. As mentioned above, there were also stick or calling-on pushbuttons, code indicators, and code cancelling buttons. Other controls, such as switches for the switch heaters used in winter, were also sometimes present. The board did not show the signal aspects as in the animation; these have been added to show you how the signals responded to the commands and changes in track occupancy.

The movement of trains was recorded on a strip chart where a pen for each OS track section (OS means "on sheet," from the traditional reports that telegraph operators made to the dispatcher to update his train sheet) made a mark when a train occupied it. An OS section was the piece of track containing the switch, or any other track circuit. The chart was 16.5" wide, and moved at 3" per hour. A 200-foot roll of paper would last a month. A pen could be provided for each of up to 40 OS sections. The dispatcher drew short lines between the dots to make a complete record. This was only a record, and was not very useful to the dispatcher in handling trains. The dispatcher signed the train sheet when going off duty.

The dispatcher also had a telephone, with a loudspeaker or, less conveniently, earphones, and could signal a train crew or signal maintainer to call him at a telephone box by displaying a light in its vicinity, usually on the relay box. This was very important, since there were no longer any operators along the line to handle messages. Now, of course, this has been replaced by continuous contact by means of radio.

A train required only a Clearance Card to proceed from its initial station; afterwards, it was governed by signals. Slow orders were delivered with the Clearance Card, which was OK'd by the train dispatcher the usual way.

When a crew was allowed to switch at a control point and operate the switch manually, or when a switch or signal was faulty, the dispatcher would hang a small flag on the lever or levers concerned to remind him of the fact. A track and time limit form was often filled out by the crew over the telephone to authorize occupation of an agreed length of the main track for an agreed length of time. A train also had to have permission to reverse, or to leave the main track at other than a controlled switch. Whatever happened, signals automatically protected any such movements. What one wanted to avoid was unexpected actions in the face of a moving train, or delay to important traffic.

To allow control a complete operating division of 100 miles or more, with perhaps twenty control points, some way had to be found to economize on wires, since one could not use direct wire control. This was first done with the one-wire system, that managed each control point with only one wire. The dispatcher had to move a peg, as in cribbage, when a train lighted an OS light as it briefly occupied a short track circuit. The machine in our example uses the later two-wire system, a modern coded system, where each control point had a unique code, and commands could be sent to any of the control points, while any control point could report conditions to the dispatcher. This machine, obviously, was much more convenient to use than the old, and provided much more information. When switches and signals were in the vicinity of the CTC board (as in many small installations) they were controlled by direct wire, not coded signals. CTC was one of the earliest uses of pulse coding in electronics, which is now widespread.

Installation of CTC was a good opportunity to upgrade, lengthen, and signal sidings so they could be used at 30 mph or even higher, and permit nonstop meets. Normally, a siding must be used at restricted speed, not faster than 15 mph, expecting to find cars on the track. A No. 15 frog and 24-foot points would allow 25 mph entering and leaving sidings. In this case, the siding was left unsignaled, and movement at restricted speed not exceeding 25 mph was allowed. The signal would then show red over yellow, Restricting, for trains to enter the siding. If a switch capable of being used at medium speed, 30 or 45 mph, was installed, and the siding was track-circuited, then the train could proceed at medium speed, and the signal would show red over green, Medium Clear. The intermediate signal governing the approach to this signal could then show Approach Medium, yellow over green, or flashing yellow, further facilitating movement. A dwarf signal was provided as a leaving signal for the sidings, authorizing movement through the switch and back onto the main line. The operating time table specified the maximum speeds for the switches, as well as the permissible speed on the sidings. Needless to say, one did not set out bad-order cars on a controlled siding! Often, a spur track was provided for this purpose.

The first long CTC installation was carried out in 1937 between Derby and Akron, Colorado, 105.3 miles, on the Chicago, Burlington and Quincy. It was chosen over double-tracking for the bottleneck that had developed between the junction at Brush and Denver. The machine, made by the Union Switch and Signal Company of Swissvale, Pennsylvania, was originally at Brush, moved to McCook in 1947, and was replaced in 1992. Twenty sidings were controlled, as well as the junction at Brush. This machine may be seen today at the Colorado Railroad Museum, Golden, Colorado. The final stretch from Denver to Derby, 5.6 miles, already had a CTC installation.

Before this, CTC had already replaced various means, such as the electric train staff or controlled manual block, for operating short difficult sections through tunnels, bridges, and gauntlet tracks at various points throughout the country, as well as at busy junctions and terminals. Direct control by the dispatcher and movement by signal indication had been used on short sections since the 1870's, when Frank Pope's remotely controlled electrical signals were used for the purpose between Sunbury and Northumberland, Pennsylvania, on the Philadelphia and Erie (PRR), but did not become practical until modern electrical technology permitted its development in the 1920's. After the Akron- Denver installation, it was used more and more on complete divisions, not simply on short sections. It is now the primary means for operating trains where the traffic is moderate to heavy, and has been further developed to permit computer control.


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Composed by J. B. Calvert
Last revised 29 May 1999