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Automatic Block Signal

Automatic Block Signal, or ABS, systems consist of a series of signals that govern blocks of track between the signals. The signals are automatically activated by the conditions of the block beyond the signal. Signals in ABS territory do not denote occupancy. Signals in ABS territory are set up to denote the most restricted indication. For instance, although train wheels 'shunt' the track, something metal may also 'shunt' the track.

Automatic block signals also detect the status of a following signal. If a signal is displaying a stop indication, the preceding signal will display an aspect that warns the train crew that the following signal may require the train to stop; such as an approach or restricting signal, denoted by yellow or red aspects, respectively.

ABS systems detect track occupancy (by a train or obstruction) by passing a low-voltage current through the track between the signals and detecting whether the circuit is closed, open, or shorted. A train's metal wheels and axles will pass current from one rail to the other, thereby shorting the circuit. If the ABS system detects that the circuit is shorted between two signals, it understands that a train, or obstruction is occupying that block and will "drop" the signals (display a restricting or stop indication) on either side of that block to prevent another train from entering (if the block is governed by a positive stop).

ABS system electronics are also able to detect breaks in the rail or improperly-lined switches (if the switch is established in the circuit), which result in an open circuit. These will also cause the signal's aspect to 'drop', preventing any trains from entering the block (if the signal system prevents it), and running the risk of a run-through switch or derailment.

Train crews that operate in ABS, often operate with track warrants or traffic control.


Single Direction ABS

The most basic form of automatic block signaling involves multiple tracks with a defined direction of traffic on each track. Single Direction ABS was frequently referred to as Rule 251 Operation for the common Rulebook entry which read something to the effect of Wayside automatic block signals shall supersede the [timetable] superiority of trains for all following movements. This in effect meant that when wayside automatic block signals were installed they would supersede the old timetable operation practice of timetable "inferior" trains needing to clear out of the path of "superior" trains. Trains would now follow each other based on signal indication.[1]

Tracks governed by Rule 251 has a designated "current of traffic" (much like a two-lane road). Trains running against the current of traffic need to still be governed by some form or train order or other absolute block authority. Due to the increasing popularity of single track operation with passing sidings as well as the basic need for operational flexibility, Rule 251 operation is gradually being phased out in favour of bi-directional CTC or Rule 261 operation.

Bi-Directional ABS (a.k.a. Centralized Traffic Control)

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.[2] 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.

A number of strategies have been implemented to solve the problems of bi-directional operation over a single track. By staggering the blocks for each direction and using more complicated signal logic problems with head-on collisions may be eliminated, but the issue with two opposing trains becoming engaged in a Mexican standoff remains. One method, popular with western railroads such as the ATSF and Southern Pacific, as well as trolley car lines was known as Absolute Permissive Block[3] or APB. APB worked by having absolute signals at the entrance of every bi-directional portion of track. When a train entered the bi-directional segment, all of the opposing signals would "tumble down" to their most restricting aspect, with the entry signal at the opposite end of the track segment displaying an absolute Stop. This would serve to automatically "reserve" an open track segment in the direction of the first train that entered it and keep the track segment reserved until it was completely clear.

APB was popular in North America throughout the first half of the 20th century as it was fully automatic and required simple logic and no form of remote control. Automatic, bi-directional single track operation was made possible without having to build expensive interlockings at the passing sidings. Trolley and interurban lines made use of spring switches at the ends of passing sidings so that trains on single track lines could pass each other at fixed points without the intervention of a central authority or expensive interlockings.

In the East, where traffic densities were higher, railroads such as the PRR would implement manual traffic control systems for bi-directional tracks. Manned interlocking towers at the ends of each line segment would have traffic control levers that would set the current of traffic for a given line segment. When the levers were set to establish a direction of traffic, opposing signals would be drop to their most restrictive aspect and no opposing movements would be able to be routed into that segment. Eventually this manual traffic control system would be improved with new telecommunications technology allowing remote operation by a single operator or dispatcher and thus became Centralized Traffic Control, the system now widely used today.

Signaling enhancements

Cab signaling Systems or CSS (also known as Automatic Cab Signaling/Automatic Speed Control, or ACS), is often used as an overlay for ABS, Rule 251 and CTC. This system provides train crews with information about the next signal indication, even if the signal mast is not visible. Automatic Train Stop, or ATS, systems provide wayside inductors that, when activated, alert the crew of conductor and engineer, that the train has passed a signal other than Clear and if the signal is not acknowledged the train's brakes will be applied. Automatic Train Control, or ATC, adds in-cab enforcement to these and will apply the brakes if a dangerous situation arises, such as when the next signal is displaying a stop indication but the crew has not begun slowing the train. Some form of ATS or ATC is required on all U.S. rail lines that operate at 80 mph or more.

A further enhancement designed to work in both signaled and dark territory is Positive Train Control, or PTC. This system is an overlay on the conventional methods of operation but also uses satellite-based tracking and computerized radio communication to verify the authority given to the train, current location of the train, the status of the next signal (if any), the position of switches (which will be equipped with a sensor and radio transmitter), and the location of any oncoming trains. As in ATC, if a dangerous situation arises, the system will apply the brakes.

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