Signalling
Brunel developed a system of "disc and crossbar" signals to control train movements, but the people operating them could only assume that each train reached the next signal without stopping unexpectedly. The world's first commercial telegraph line was installed along the 13 miles (21 km) from Paddington to West Drayton and came into operation on 9 April 1839. This later spread throughout the system and allowed stations to use telegraphic messages to tell the people operating the signals when each train arrived safely. A long list of code words were developed to help make messages both quick to send and clear in meaning.
More conventional semaphore signals replaced the discs and crossbars over time. The GWR persisted with the lower quadrant form, where a "proceed" aspect is indicated by lowering the signal arm, despite other British railways changing to an upper quadrant form. Electric light signals of the "searchlight" pattern were later introduced at busy stations; these could show the same red/green or yellow/green aspects that semaphore signals showed at night. An "automatic train control" system was introduced from 1906 which was a safety system that applied a train's brakes if it passed a danger signal.
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