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Centralized traffic control (CTC) is a signalling system used by railroads. The system consists of a centralized train dispatcher's office that controls railroad switches in the CTC territory and the signals that railroad engineers must obey in order to keep the traffic moving safely and smoothly across the railroad. In the dispatcher's office is a graphical depiction of the railroad on which the dispatcher can keep track of trains' locations across the territory that the dispatcher controls. Larger railroads may have multiple dispatcher's offices and even multiple dispatchers for each operating division.


Development and technology

In general, most railroads had their own set of operating instructions with respect to how trains were to operate over their system. In some cases a rail crew might operate on a foreign carrier's lines, and would be required to also know their rules. To ease this situation, a number of railroads would use a "consolidated code," or set of common operating instructions that were the same for all of the railroads using those. One of the more common rules from this consolidated code was Rule 251, which read as follows:

Key to the concept of CTC is the notion of Traffic Control as it applies to railroads. In single direction "Rule 251" operation, each section of track has a timetable defined flow of traffic. Trains running against the flow of traffic need to be protected by special procedures usually involving some form of absolute block. Implementing a Rule 251 line is very straight forward as the logic for the signaling system is very simple. One might assume that making a track bi-directional would be no harder than wiring in a separate signal direction system in reverse, but this is not the case.

A bi-directional rail line also needs to avoid the situation of two trains approaching each other on the same section of track. A bi-directional Rule 251 type system would allow trains to encounter each other head on, and while most trains would end up approaching one another at restricted speed and, in theory allow for a safe stop, the case would exist of one or both trains receiving a yellow or "Approach" signal and therefore assuming the next signal block is unoccupied when in fact there is traffic approaching head on. While the safety issues can be solved with signal block overlaps and other tricks, you will still end up with trains on the same track in a Mexican standoff requiring one train to back up to the nearest passing point.

Before the advent of CTC there were a number of solutions to this problem. Many western railroads used an automatic system called absolute permissive block where trains entering a stretch of double track would cause all of the opposing signals between there and the next passing point to "tumble down" to a Stop position this preventing opposing trains from entering. In areas of higher traffic density sometimes bi-directional operation would be established between manned interlocking towers. Each section of bi-directional track would have a traffic control lever associated with it to establish the direction of traffic on that track. Often, both towers would need to set their traffic levers in the same way before a direction of travel could be established. Block signals in the direction of travel would display according to track conditions and signals against the flow of traffic would always be set to their most restrictive aspect. Furthermore, no train could be routed into a section of track against its flow of traffic and the traffic levers would not be able to be changed until the track section was clear of trains.

The use of manned interlocking towers to handle bi-directional operations on low density, single track lines with passing sidings was too costly to be economically feasible and to solve this problem the General Railway Signalmarker company developed Centralized traffic control (CTC) technology. Its first installation in 1927 was on a 40-mile stretch of the New York Central Railroad between Stanley and Berwick Ohiomarker, with the CTC control machine located at Fostoria, Ohiomarker. .

CTC was designed to enable the train dispatcher to control train movements directly, bypassing local operators and eliminating written train orders. Instead, the train dispatcher could directly see the trains' locations and efficiently control the train's movements by displaying signals and controlling switches. It was also designed to enhance safety by detecting track occupancy and automatically preventing trains from entering a track against the established flow of traffic.

The basic component of a CTC system is detecting track condition and occupancy. The track at either end of the signal block is electrically insulated, and within the block a small electrical current passes through the track. When a train passes a signal and enters a block, the metal wheels and axle of the train short-circuit the current, which causes a relay associated with the track circuit to itself become de-energized (see track circuit and rail circuits). Additionally, any fault in the rail or failure in the signal system, such as a broken rail, a cut wire, or a power failure, will cause the relay to de-energize. When this relay is de-energized, the system understands the track to be occupied or damaged, and the signals show it as such to prevent a train from proceeding and encountering harm.

What made CTC machines different from standard interlocking machines was that the vital interlocking hardware was located at the remote location and the CTC machine only displayed track state and sent commands to the remote locations. A command to display a signal would require the remote interlocking to set the flow of traffic and check for a clear route through the interlocking. If a command could not be carried out due to the interlocking logic, the display would not change on the CTC machine. Initially the communication was accomplished by dedicated wires or wire pairs, but later this was supplanted by pulse code systems utilizing a single common communications link and relay-based telecommunications technology similar to that used in crossbar switch. Also, instead of only displaying information about trains approaching and passing through interlockings, the CTC machine displayed the status of every block between interlockings where previously this territory had been effectively a black hole as far as the dispatcher was concerned. The CTC system would allow the flow of traffic to be set over many sections of track by a single person at a single location as well as control of switches and signals at interlockings, which also came to be referred to as control points.

CTC machines started out as small consoles in existing towers only operating a few nearby remote interlockings and then grew to control more and more territory, allowing less trafficked towers to be closed. Over time the machines were moved directly into dispatcher offices, eliminating the need for dispatchers to first communicate with block operators as middlemen. In addition the electromechanical control and display systems have been replaced with computer operated displays.

Signals and signal blocks

The most prominent feature of CTC is its signals. Signals govern movement over the section of track, or signal block, between that signal and the following signal.

When calculating the size of the blocks and, therefore, the spacing between the signals, the following has to be taken into account:
  • Track speed (the maximum speed the train is allowed to travel)
  • Gradient (to compensate for the assistance or otherwise afforded to deceleration)
  • The braking characteristics of the train(s) that travel on that line
  • Sighting (the ability of the engineer to see the signal)
  • Reaction time (of the engineer)

A signal is placed where signal blocks meet. Separate signals are placed for trains traveling in opposite directions. Signals are generally placed on the right side of the track; however, opposing signals may both be mounted on the same signal mast in opposite directions or may be located on an overhead support system.

These signals are one of two types: an absolute signal, which is directly controlled by the train dispatcher and is located at a control point, or an intermediate signal, which is automatically controlled by the conditions of the track in that signal's block and by the condition of the following signal. Train dispatchers cannot directly control intermediate signals.

Signals have aspects and indications. The aspect is the visual appearance of the signal; the indication is the meaning.

Switches and control points

The majority of control points are located at electronically-operated switches. These switches are called dual-controlled switches, as they may be either remotely controlled by the train dispatcher or by manually operating a lever or pump on the switch mechanism itself (although the train dispatcher's permission is generally required to do so). These switches may lead to a passing siding, or they may take the form of a crossover, which allows movement to an adjacent track.

Sidings are located at stations. A station is a place along the railroad designated by name in the railroad's timetable, which is a publication with instructions governing train movements--as opposed to a passenger timetable, which details the arrival and departure times of passenger trains--and does not necessarily refer to a place where a passenger train stops to allow passengers to get on or off.


Although some railroads still rely on older, simpler electronic lighted displays and manual controls, in modern implementations, dispatchers rely on computerized systems similar to SCADA systems to view the location of trains and the aspect, or display, of absolute signals. Typically, these control machines will prevent the dispatcher from giving two trains conflicting authority without needing to first have the command fail at the remote interlocking. Modern computer systems generally display a highly simplified mock-up of the track, displaying the locations of absolute signals and sidings. Track occupancy is displayed via bold or colored lines overlaying the track display, along with tags to identify the train (usually the number of the lead locomotive). Signals which the dispatcher can control are represented as either at Stop (typically red) or "displayed" (typically green). A displayed signal is one which is not displaying Stop and the exact aspect that the crew sees is not reported to the dispatcher.

By country


The first CTC installation in Australia was commissioned in September 1957 on the Glen Waverley line in suburban Melbournemarker. in length, the Victorian Railways installed it as a prototype for the North East standard project. CTC has since been widely deployed to major interstate railway lines.

United States

CTC-controlled track is significantly more expensive to build than non-signalled track, due to the electronics and failsafes required. CTC is generally implemented in high-traffic areas where the reduced operating cost from increased traffic density and time savings outweigh the capital cost. Most of BNSF Railway's and Union Pacific Railroad's track operates under CTC; the portions that are not are generally lighter-traffic lines that are operated under Track Warrant Control (BNSF and UP) or Direct Traffic Control (UP).

Recently the costs of CTC has fallen as new technologies such as microwave, satellite and rail based data links have eliminated the need for wire pole lines or fiber optic links. These systems are starting to be called train management systems.


There are several companies offering individual components as well as turnkey systems that comprise the elements of a CTC system. These suppliers include:

Complete systems

Control office equipment

Field equipment

See also


External links

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