Automatic block signaling, or ABS, is a railroad communications system that consists of a series of signals that divide a railway line into a series of sections, or "blocks". The system controls the movement of trains between the blocks using 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. The introduction of ABS reduced railways' costs and increased their capacity. Older manual block systems required human operators. The automatic operation comes from the system's ability to detect whether blocks are occupied or otherwise obstructed, and to convey that information to approaching trains. The system operates without any outside intervention, unlike more modern traffic control systems that require external control to establish a flow of traffic.
The earliest way of managing multiple trains on one track was by use of a timetable and passing sidings. One train waited upon another, according to the instructions in the timetable, but if a train was delayed for any reason, all other trains might be delayed, waiting for it to appear at the proper place where they could pass safely. Operation of trains by timetable alone was supplemented by telegraphed train orders beginning in 1854 on the Erie Railroad. A railroad company dispatcher would send train orders to stations manned by telegraphers, who wrote them down on standardized forms and handed them to train crews as they passed the station.
A manual block system in the United States was implemented by the Pennsylvania Railroad about 1863, a couple of decades before other American railroads began using it. This system required a railroad employee stationed at each signal to set the signals according to instructions received by telegraph from dispatchers. English railroads also used a "controlled manual" block system, which was adapted for use in the U.S. by the New York Central and Hudson River Railroad in 1882.
Automatic block signaling was invented in America, and first put to use by the Eastern Railroad of Massachusetts in 1871, and was soon adopted by other New England railroads. However, the cost of signals, equipment, and installation was very high in the 19th century, which deterred many railroads from installing it except on highly trafficked lines used by passenger trains. By 1906, the Interstate Commerce Commission reported that of the 194,726.6 miles of railroad in the United States, there were 41,916.3 miles protected by the manual block system, and only 6,826.9 miles of automatic block, on either single or double track. The rest of the nation's trackage was operated by timetable and train order.
However, as time went on, many railroads came to see automatic block signaling as cost effective, since it reduced the need for employees to manually operate each signal, reduced the repair costs and damage claims resulting from collisions, made possible a more efficient flow of trains, reduced the number of hours trains and crews sat idle, and decreased overall transit times from point to point.
Most ABS systems use three- or four-block arrangements, where an obstruction in the first block will prompt a warning upon entering the second block, and allow full speed for trains entering the third. Where blocks are short or higher capacity is needed, four or more blocks are used; trains are then given multiple warnings of an impending obstruction. For basic block status, the red/yellow/green system of signalling is nearly universal, with red indicating an obstructed block, yellow indicating that an obstructed block is ahead, 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 through the use of electrical track circuits. A low-voltage current is sent through the track between the signals and is detected to determine 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 (shunting) 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 track switches (if the switch is established in the circuit), which result in an open circuit. These will also change the signal indication, preventing any trains from entering the block (if the signal system prevents it), and risking running through a switch, or a derailment.
The most common forms 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 supersede 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 installed, 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 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.
A train proceeding under bi-direction ABS will cause signal indications to change to stop for a train coming the other way. It is possible that, depending on when he passed each signal, an engineer may see a stop indication while traveling at unrestricted speed from the previous signal still displaying a clear indication. To properly protect trains from these types of collisions or any situation where opposing trains can meet away from a passing siding, bi-directional ABS must be overlaid with a traffic control system such as timetable and train order, or more recently, track warrant control. These trains provide trains with conflict free movement authority that is unable to be conveyed by basic automatic block signals. Additional protection against collision may be provided through the use of staggered signals or multiple blocks held at red for opposing movements.
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.
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.