John Speller's Web Pages Automatic Block Signaling

John Speller's Web Pages - US Railroads

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The American system of Automatic Block Signaling (ABS) developed from around 1872 onwards and depends on track circuiting to monitor the location of trains and use their locations to operate the signals automatically. A train will actuate the track circuit, which will actuate an electro-magnetic relay, which in turn will actuate the signal electrically. It is normal to have two signals, one above the other, and to track the train for four blocks ahead. A train occupying a block will show red above red and requires the train to stop, a train in the next block will show yellow above red and requires the train to slow, a train two blocks ahead will show yellow above yellow, and a train three blocks ahead will show a clear signal of green above green. An additional protection is often provided by Automatic Train Control (ATC), which sends a signal to brake the train if the signal is adverse. Since 1947 trains running on lines with automatic block signaling are normally allowed to travel at a maximum speed of 79 m.p.h.

George Westinghouse bought and merged the Union Electric Signal Company, founded by William Robinson, inventor of the track circuit, with the Interlocking Switch & Signal Company,which had pioneered interlocking of signalsand switches, as the Union Switch & Signal Company in 1881. Union Switch & Signal operated independently until 1917, when it was taken over by the Westinghouse Air Brake Company. In 1925 the firm acquired the Hall Signal Company, primarily to obtain the latter company's patents for searchlight signals. US&S was subsequently taken over by American Standard in 1968. The searchlight signals produced by Union Switch and Signal were long the main stay of utomatic Block Signaling. They consisted of a powerful searchlight with moveable lenses allowing for red, green and yellow lights. Some US&S searchlight signals were used on the Great Western and London & North Eastern Railways in Great Britain, but without making use of the Automatic Block Signaling system.

Searchlight signal on the former Lehigh Valley Railroad near Mauch Chunk, Pennsylvania. Image courtesy of Wikipedia Commons
Mechanism of a Union Switch & Signal single lens multiple aspect searchlight signal. The US&S Company was founded by George Westinghouse and became part of the Westinghouse Air Brake Company in 1917. In England, similar signals were used on the Great Western Railway at Paddington, Cardiff and Bristol. Image courtesy of Wikipedia Commons
An array of searchlight signals on the Delaware and Hudson Railroad. This is an enhanced system using three rather than two searchlight signals per track. Image courtesy of Wikipedia Commons
Dwarf searchlight signals on the CSX Boston line. The "C" signal is for the benefit of trains not fitted with cab signalling. Image courtesy of Wikipedia Commons
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