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When a train is waiting at a signal it is "in rear of" that signal and the danger being protected by the signal is "in advance of" the train and signal. In North American practice, a distinction must be made between absolute signals, which can display a "Stop" (or "Stop and Stay") indication, and permissive signals, which display a "Stop ...
Signals are most commonly mounted on trackside masts about 12 to 15 feet (3.7 to 4.6 m) high to put them in the eyeline of the engineer. Signals can also be mounted on signal bridges or cantilever masts spanning multiple tracks. Signal bridges and masts typically provide at least 20 feet (6.1 m) of clearance over the top of the rail.
The first such systems were installed on an experimental basis in the 1910s in the United Kingdom, in the 1920s in the United States, and in the Netherlands in the 1940s. . Modern high-speed rail systems such as those in Japan, France, and Germany were all designed from the start to use in-cab signalling due to the impracticality of sighting wayside signals at the new higher train spee
In the very early days of railways, men (originally called 'policemen', which is the origin of UK signalmen being referred to as "bob", "bobby" or "officer", when train-crew are speaking to them via a signal telephone) were employed to stand at intervals ("blocks") along the line with a stopwatch and use hand signals to inform train drivers ...
The RTC can also issue special permissions to trains via radio. In order to pass signals set at stop (Rule 564), reverse direction within a block (Rule 577) or enter the main line at a manual switch not equipped with a signal (Rule 568), the train crew must copy the RTC's instructions and repeat them back correctly before being allowed to proceed.
The first reached by a train is known as the home signal. The last stop signal, known as the starting or section signal, is usually located past the points etc. and controls entry to the block section ahead. The distance between the home and starting signals is usually quite short (typically a few hundred yards), and allows a train to wait for ...
As train speeds increased, it became increasingly difficult for enginemen to see hand signals given by the policemen, so the railways provided various types of fixed signals to do the job, operated by the policemen, or signalmen as they soon became known (it is due to this that British railway slang still names signalmen as "Bobbies"). [4]
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. [3] [4]: 6–9