Find out more about Signalling on our railways via several Museum Exhibits:
A NEW Museum display explores how me made our railways safer to travellers.
In the days of the first British railways, “policemen” were employed by every railway company. Their jobs were many and varied, but one of their key roles was the giving of hand signals to inform engine drivers as to the state of the line ahead. They had no means of communication with their colleagues along the line, and trains were only protected by a time interval; after a train had passed him, a policeman would stop any following train if it arrived within (say) 5 minutes; for any between 5 and 10 minutes after, he would show a caution signal, and after 10 minutes, the line was assumed to be clear. Therefore, if a train failed midsection (as was very common in the early days), the policeman controlling entry to the section would not know, and could easily give a ‘clear’ signal to a following train when the section was not in fact clear. The number of collisions which resulted from this led to the gradual introduction of the absolute block principle; all systems of working other than this (including time-interval and permissive block) were outlawed on passenger lines in 1889, and all passenger lines were suitably equipped by 1895.
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. Many types were devised, but the most successful was the semaphore, introduced in 1841 and soon becoming widespread, although some other types did linger on until the 1890s.
The traditional British signal is the semaphore, comprising a mechanical arm that rises or drops to indicate ‘clear’ (termed an “upper-quadrant” or “lower-quadrant” signal, respectively). Both types are fail safe in the event of breakage of the operating wire but lower-quadrant signals require a heavy counter-weight (usually in the form of the “spectacle” that carries the coloured lenses for use at night) to do that, while upper-quadrant signals return to “danger” under the weight of the arm.
During the 1870s, almost all the British railway companies standardised on the use of semaphore signals.
There are two main types of semaphore; stop and distant. The stop signal consists of a red, square-ended arm, with a vertical white stripe typically 9-12 inches (230–300 mm) from the end, and advises the driver whether the line immediately ahead is clear or not. A stop signal must not be passed in the horizontal “on” (danger) position, except where specially authorised by the signalman’s instruction. By night, it shows a red light when “on” and a green light when “off” (clear). The green light is usually produced through the use of a blue spectacle lens, which produces green when lit from behind by the yellowish flame from a paraffin lamp.
The other type is the distant signal, which has a yellow arm with a ‘V’ (“fishtail”) notch cut out of the end and a black chevron typically 9-12 inches (230–300 mm) from the end. Its purpose is to advise the driver of the state of the following stop signal(s); it may be passed in the “on” position, but the driver must slow his train to be able to stop at the next stop signal. When “off”, a distant signal tells the driver that all the following stop signals of the signal box are also “off”, and when “on” tells the driver that one or more of these signals is likely to be at danger. By night, it shows a yellow light when “on” and a green light when “off”.
Current British practice mandates that semaphore signals, both upper and lower quadrant types, are inclined at an angle of 45° from horizontal to display an “off” indication.
The principle of the British Railways Absolute Block system is quite straightforward. Only one train should be in any one block section at a time. The Absolute Block system only applies on double or multiple lines where trains always use each line in a pre-determined direction. Separate regulations apply to single and bi-directional lines.
A block section is a section of line, stretching from the last stop signal controlled by one signal box to the first stop signal controlled by the next signal box. The terms first and last refer to the order they are seen by a driver as he travels along the line.
Read even more: https://www.signalbox.org/