Automatic block signaling

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.

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