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Automatic Block Signaling, or ABS, is a block system that consists of a series of signals that divide a railway line into a series of blocks and then functions to control the movement of trains between them through automatic signals. ABS operation is designed to allow trains operating in the same direction to follow each other in a safe manner without risk of rear end collision while reducing costs and increasing capacity from previous manual block systems that required human operators. The automatic operation comes from an ability to detect if blocks are occupied or otherwise obstructed and then convey that information to approaching trains. The term "Automatic" in ABS refers to the operation of the system without any outside intervention and contrasts with more modern traffic control systems that require external control to establish a flow of traffic.
Basic operation 
Most ABS systems use three or four block arrangements where an obstruction in block 1 will prompt a warning entering block 2 and allow full speed for trains entering block 3. In instances where blocks are short or higher capacity is needed 4 (or more) blocks are used where trains are then given multiple warnings of an impending obstruction. For basic block status the Red/Yellow/Green system is nearly universal with Red indicating an obstructed block, yellow indicating an obstructed block is approaching and green indicating that no obstruction is to be expected.
The most common way that ABS systems detect track occupancy (by a train or obstruction) is 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). In the United Kingdom the system is referred to as Track Circuit Block (TCB) to avoid confusion with the use, in that country, of the acronym AB for Absolute Block.
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.
Single direction ABS 
The most common form of ABS was implemented on double track rail lines in high density areas that has exceeded the capacity provided by either timetable and train order or other manual forms of signaling. ABS would be set up in such a way to cover train movements only in a single direction for each track. The movement of trains running in that direction would be governed by the automatic block signals which would superseded the normal superiority of trains, where such systems applied. Movement of trains operating against the established flow of traffic would still require train orders or other special "manual" protections to prevent a collision. Therefore under ABS operation trains moving in the "wrong" direction is an uncommon occurrence and may be not well supported by the track infrastructure.
Aside from increased capacity and safety compared to a manual block system, another advantage of the single direction ABS system is that it is relatively inexpensive to install and operate, each signaling point requiring only one relay for its own track circuit and one additional relay for each block in its control length. Where interlockings are fitted entrance from the reverse direction may be fitted with less expensive dwarf or shunt signals and any non-interlocked points may be fitted in a trailing configuration, which require less safety equipment. Traditional ABS also has no single point of failure and no requirement to transmit state information to a central location and as long as its own track circuit is unoccupied and the inputs from the block or two down the line report the same, and ABS signal will display a clear indication.
Another feature of single direction ABS lines is the use of non-interlocked crossovers to facilitate wrong direction running or trains reversing direction away from a traditional terminal. Operated by hand or from a ground frame, these facilities made use of track circuit shunts and time locked points to ensure that any approaching train would be brought to a standstill before encountering the obstruction. While reversing direction onto the other track required no outside intervention, running against the defined current of traffic would require train orders or the services of a pilotman and such ABS crossovers would feature either telephone communications with the train dispatcher or a structure that could serve as a temporary block station.
Bi-directional ABS 
Bi-directional ABS is distinct from other forms of bi-directional block systems such as Centralized traffic control as it has no capability to set or enforce a flow of traffic. As in single direction ABS, the signals act only in accordance to track occupancy with the only difference being the ability to apply this logic in both directions. This means that the default indication for any bi-directional ABS signal would be Clear in both directions.
The result of this is that it is possible for two trains to enter the same single track segment in opposing directions. At best this will result in a Mexican standoff with one train then needing to back up to the closest passing point and at worst a head on collision. Therefore bi-directional ABS requires some external form of traffic control either in the form of train orders or a modern equivalent such as Track Warrant Control.
Bi-directional ABS was popular on lower density single track lines that nevertheless needed an upgrade from older train order or manual block systems. Bi-directional ABS was placed in service with the same passing siding infrastructure that operated under the older plain train order system. Opposing trains would still need to clear the main track based on their superiority or instructions, however, unlike under the older system, trains moving in the same direction could follow one another based on signal indication. This eliminated the need for some of the more cumbersome flagging procedures to protect the rear of trains working under the timetable and train order system.
Automatic Traffic Control 
Several systems were implemented as a hybrid of bi-directional ABS and signaling systems with traffic control functions. Most of these fell along the general heading of Absolute Permissive Block where a typical bi-directional ABS would be outfitted with signaled passing points with absolute signals protecting the next segment of single track. Trains wishing to enter the single track segment would be automatically detected at the passing point and if the segment was free from opposing movements, signaled to proceed. Trains wishing to enter in the opposite direction would be held until the entire segment was again clear of opposing traffic. Trains working under such a system would tend to still follow the bi-directional ABS rules although without the risk of trains meeting head on.
These types of systems were also very popular Interurban streetcar lines due to the high capacity and low cost.
As the cost of electronics and signaling hardware fell with respect to the cost of labor, Automatic Block Signaling began to be replaced by Centralized Traffic Control and other systems that allowed trains to run in any direction on any track. Traffic control systems not only make use of bi-directional signaling, but also prevent trains being routed against the set flow of traffic at interlockings and automatically reduce all wrong direction automatic signals to an obstructed block state. This completely eliminates manual traffic setting procedures in bi-directional ABS schemes and wrong direction contingency procedures in single direction schemes.
See also 
- "Railroad's traffic control systems". 2006-05-01. Retrieved 2010-04-08.