Home signal
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In railway signalling, a home signal (British English and sometimes in American English as well) or absolute signal (US English) is a signal that protects junctions, points (switches in American terminology), movable bridges, platforms, or block sections.
The technical term in the UK is stop signal, referring to any signal whose most restrictive indication is Danger (and which compels a stop). Signals that warn of stop signals but which don't compel a stop when in their most restrictive position are referred to as distant signals.
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[edit] Types of stop signal
Using the UK definition, home signal is but one kind of stop signal. A given signal box may have one or more stop signals on each running line. In a traditional mechanically signalled area, it is most common for a signal box to have two stop signals governing each line. The first reached by a train is known as the home signal — it protects points, junctions, level crossings, etc. controlled by the box. The second stop signal — known as the starting or Section signal is located past the points, junctions, and so forth, and protects the entrance to the block section ahead.
A signal box controlling a complex track arrangement may have a number of stop signals on each line. Typical names are outer home, inner home, starting, advanced starting, and so forth.
The absolute block signalling regulations provide that a train may not be accepted from a signal box in the rear where the line is occupied or fouled to the clearing point — usually a point 440 yards (0.25 mile) ahead of the first stop signal controlled by a box. This zone of protection allows for an error in judging a train's braking and was particularly important in the days when unbraked goods wagons were common.
Shunting movements that take place between the home signal and clearing point therefore prevent the acceptance of a train from the box to the rear; at locations where this would cause delays, a second home signal — referred to as the outer home is placed at least 440 yards to the rear of the original home signal (now the inner home). With this change, the shunting movement no longer fouls the line ahead of the first stop signal, allowing a train to be accepted from the box to the rear. Any approaching train will, of course, be checked by the distant signal at Caution and stopped at the outer home at Danger unless the shunting movement by that time has cleared the line ahead of the inner home signal.
An advanced starting signal might be used at a location where it might be desirable to advance a train from a station platform before the section ahead becomes available. In this scenario, the starting signal permits the train to draw forward from the station area toward the advanced starting signal, which controls entry to the section ahead.
If any of the signals beyond the first stop signal are at danger, the previous signals will also be held at danger until the train is almost at a stand, to indicate to the driver that the next signal is at danger. In many cases, a berth track circuit will be provided just before the signal, and if following signals are at danger, the signal will be electrically locked until the train has occupied the track circuit.
In North America, the foregoing terminology was not used, as the development of American signalling practice diverged from that in the United Kingdom during the late nineteenth century. In America, where the term home signal is in common usage, it generally refers to the "generic" British definition of stop signal — namely any signal whose most restrictive indication is Stop (Danger).
[edit] Relationship between distant signals and stop signals
Trains travelling at any significant speed require hundreds if not thousands of metres to stop. In order that a train can be brought to a stand before reaching a home signal at Danger, it is necessary to have a distant signal located at at least braking distance from the succeeding stop signal.
The driver of a train encountering a distant signal at Caution must expect the stop signal (home signal) to be at Danger and must adjust the train's speed so as to bring the train to a stand before reaching the stop signal at Danger. The driver of a train encountering a distant signal in the Clear position knows that all stop signals controlled by the same signal box are in the Clear position.
This is enforced by interlocking; the distant signal is prevented from assuming the Clear position until all stop signals controlled by the signal box display Clear.
[edit] Shapes and colours
Home signals vary around the world, in shape, colour, illumination, and behaviour.
The most common arrangement is a semaphore arm painted red with a white stripe to facilitate sighting, which shows a red light in the stop position, and a green light in the clear position.
The home signal arm is traditionally square-ended, which distinguishes it from the "V"-notched "fishtail" arm of a distant signal. However, some arms now have a pointed end (the opposite of a fishtail), which indicates "stop and then continue, be prepared to stop if necessary" (heavy freight trans are often permitted to continue slowly without actually coming to a complete halt).
The red colour is a good colour for stop signals since relative few backgrounds are red, and the white stripe provides an alternative should the background have an unsuitable contrast. Where particularly poor background contrast can't be avoided, a sighting board is placed behind the signal to provide adequate contrast to improve visibility.
Arrangements are made so a home signal shows a red light in the stop position and a green light in the clear position at night. This usually takes the form of a spectacle plate that was attached to the signal arm with a red and blue coloured glass. This would be illuminated from behind with either an oil lamp or an incandescent lamp run at a lower voltage than usual. When the signal was "off" the blue glass would appear green because of the lamp's spectral content.
Upper-quadrant semaphores (where the "proceed" indication is above the horizontal) have largely replaced the lower-quadrant signal shown in the illustration. This is because if the signal mechanism fails in a manner that allows the signal arm to drop, the result in a "stop" position rather than a "proceed" position.
A later development was the three-position semaphore signal. These work in the upper quadrant to distinguish them from the traditional two-position lower quadrant semaphores. When the arm is inclined upwards at 45 degrees, the meaning is "caution" and the arm in the vertical position means "clear". Thus, three indications can be conveyed with just one arm and without the need for a distant arm on the same post. The early abandonment of the three-position semaphore signal in the UK allowed for the widespread replacement of two-position lower quadrant signals by two-position upper quadrant signals.
British practice mandates that lower quadrant signals display an "off" aspect if they decline at 60 degrees. Upper quadrant signals display an "off" aspect if they incline at 45 degrees.
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