Automatic Block Signal

= Basic Automatic Block Signals =

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, and 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. [ [http://broadway.pennsyrr.com/Rail/Prr/Bor1956/rulebook1956.html#signal_rules PRR Book of Rules 1956/64 ] ]

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)

Key to the concept of Centralized Traffic Control (aka 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. [ [http://www.signalbox.org/overseas/usa/index.htm Towers in the USA ] ] 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. [ [http://www.lundsten.dk/us_signaling/abs_apb/index.html#prot_appr Railroad Signalling: APB ] ]

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. [ [http://www.lundsten.dk/us_signaling/abs_apb/index.html Railroad Signalling: APB ] ] 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. [ [http://broadway.pennsyrr.com/rail/Signal/operator_duties.html Operator Duties ] ]

Requiring manned interlocking towers at the ends of bi-directional territory was costly and labor intensive and on low density, single track lines with passing sidings it would be economically infeasible. In the 1920s technology had progressed to a point where the traffic control operations could be carried out from a centralized location and in 1927 the first Centralized Traffic Control was installed on a 40-mile stretch of the New York Central Railroad between Stanley and Berwick Ohio, with the CTC control machine located at Fostoria, Ohio [ [http://mysite.du.edu/~jcalvert/railway/ctc.htm Centralized Traffic Control ] ] .

Today, all large or heavily-trafficked railways use CTC. In CTC, train dispatchers monitor the location of trains electronically using track circuits, direct traffic by controlling the indication of absolute signals, and efficiently arrange oncoming train meets by automatically lining electronically-operated ("dual controlled") switches to cause trains to enter sidings. In addition to absolute signals controlled by the dispatcher, CTC territory usually includes intermediate signals between the absolute signals that operate similarly to ABS signals. These signals help keep trains separated between absolute signals, which can often be dozens of miles apart.

Because of the electronics used (the signals, the electronic switches, and the communications infrastructure), CTC track is much more expensive to build and maintain than track used in other methods of operation. In other methods of operation, train crews must stop to line switches when entering a siding (and may often be required to line them back when the entire train has left the main track, causing members of the train crew to walk long distances, unless otherwise authorized by the train dispatcher). CTC also enhances safety, as it allows train dispatchers to monitor the movement of trains to ensure they do not overrun their authority and provides for detecting the condition of the track.

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. [ [http://www.fra.dot.gov/us/content/784 Positive Train Control: Intelligent Railroad Systems] , Federal Railroad Administration, retrieved August 16, 2006] [ [http://www.ntsb.gov/Recs/mostwanted/positive_train.htm Most Wanted List of Transportation Safety Improvements] , National Transportation Safety Board, retrieved August 16, 2006] [ [http://www.findarticles.com/p/articles/mi_m0BQQ/is_3_44/ai_114629906 BNSF starts positive train control trial - North American Viewpoint] , by William Vantuono, "International Railway Journal", March, 2004, retrieved August 16, 2006]

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