Centralized traffic control

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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 railroad's 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.

1 Layout
2 Operation
3 Applications in the U.S.
4 Suppliers
5 See also
6 External links

[edit] Layout

[edit] Development and technology

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 signal blocks already occupied by other trains (see interlocking).

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 circuit.) 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.

The other

[edit] 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.

[edit] Signal mounting

Line side signals need to be mounted in proximity to the track which they control. When a single track is involved, the signal is normally mounted on a mast which displays the signal head at some height above the track, in order to allow it to be seen at a distance. The signal is normally put on the engineer's or driver's side of the track.

When multiple tracks are involved, or where space does not permit mast mounting, other forms are found. In double track territory one may find two signals mounted side by side on a bracket which itself is mounted on a mast. The left hand signal then controls the left hand track, and the right signal the right hand track. For mutiple tracks, a signal bridge is often used. This consists of a platform extending over the tracks; the signals are mounted on the platform directly over the tracks they control.

In some situations there is insufficient room for a mast or bridge, and dwarf signals are used. These are smaller and hold the signal head at ground level. These are sometimes seen in congested terminals and passenger depots, where many tracks meet.

[edit] 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.

[edit] Operation

Although some railroads still rely on older, simpler electronic lighted displays and manual controls, in modern implementations, dispatchers rely on computerized 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. 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). The signal aspects are represented by colored circles corresponding to the signals that train crews see.

Absolute signals at a switch contain two signal heads, one on top of the other. The top signal governs movement over the straight or main track, and the bottom signal governs movement over the diverging route.

Normally, signal blocks touch each other. However, control points located at switches do not touch; instead, the area between the opposing signals is all considered to be the control point, and track occupancy in the control point is displayed separately on the control machine.

[edit] Applications in the U.S.

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).

East of the Mississippi River, CTC is generally applied only to single track railway with passing sidings; double-track railway is generally operated under Rule 251. In the western United States, most signaled track, whether single, double, triple or more is generally operated under CTC. This allows the dispatcher greater flexibility in managing traffic by having some trains "cross over" to another main track to pass slower or stopped trains or allow access by trains moving in both directions to spurs and industries on both sides of the track.

[edit] Suppliers

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