No fixed signals were installed when the Great Western line was built in 1838. Instead, the line was divided into "beats" and patrolled by the company's policemen. Trains were controlled using hand signals, much the same way as modern traffic police direct motor traffic. Small "sentry" boxes (with no doors) were provided for the policemen. These boxes were pivoted at the base so the door could be turned away from the wind.
In 1840, the first fixed signals were installed at stations and level crossings. These consisted of a ball or disc hoisted up the post to indicate "all clear". Trains were not permitted to pass the signal post if there was no disc displayed.
The early fixed signals were soon superceded by "disc and crossbar" signals which were introduced in 1841. They were one of the first signals in the country to have positive indications for both "all clear" and "stop". These signals were on posts between 40 and 60 feet (12.3m - 18.5m) tall and consisted of a 3ft or 4ft diameter disk mounted over an 8ft x 1ft 3in horizontal bar set a right angles to the disk. Both disk and crossbar where made of metal and were perforated to reduce wind resistance. "All clear" was signalled by turning the disc to face the train, and "stop" indicated by turning the post a quarter turn so the bar faced oncoming traffic. While semaphore signals gradually replaced disk and crossbar signals from 1865 onwards, a few disc and crossbar signals survived until the 1900s.
While the Great Western Railway made some early trials with electric telegraph for communicating between stations, it was not until 1877 that trains were controlled by telegraph. Until 1877, trains were controlled using a time interval process which operated by not permitting a train to follow another for a set period of time. While disc and crossbar signals controlled stations, junctions and level crossings, the time interval between trains was controlled by "fantail" signals. Like the disc and cross bar signals, these arrow shaped signals (so called after a short lived fan shaped version of the signal) operated by turning the post. They had three settings - a green arrow pointing right to left indicated "caution", a red arrow pointing left to right indicated "stop" and the arrow pointing parallel to the line indicated "all clear". At night red, green and white lights were used to indicate "stop", "caution" and "all clear" respectively. Initially the signal was set to "stop" for 3 minutes after a train passed, and then set to caution for a further 7 minutes. Trains were allowed to proceed past the "caution" signal at reduced speed. In 1852, as trains became heavier and faster, the "stop" and "caution" time intervals were respectively changed to 5 and 5 minutes after passenger trains and 8 and 7 minutes after goods trains.
From 1865 both disc and cross bar and fantail signals were replaced with semaphore signals which initially had three positions - horizontal indicated "stop", angled at 45 degrees indicated "caution" and vertical (hidden inside the post) indicated "all clear". These were later replaced with signals with only two positions - the angled position meaning "all clear".