Distant signal
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Overview
In railway signalling, a distant signal (British English) or approach signal (US English) is a signal that repeats the aspects of a signal some distance ahead.
Trains travelling at any significant speed require hundreds if not thousands of metres to stop, and by the time that an obstruction is reached it will be too late to stop.
It is therefore necessary to have a home signal at the obstruction, and a Distant signal located at at least braking distance on the approach side.
Shapes and colours
Distant signals vary around the world, in shape, colour, coloured lights at night, and how they work.
The most common arangement is a semaphore arm painted yellow with a black stripe to facilitate sighting, which shows a yellow light in the warning position, and a green light in the clear position.
The distant signal has a fishtail shape to distinguish it from a Home signal.
The yellow colour (used since 1922) is a good colour for warnings.
The black "V" on the arm is to help recognise the signal when the background is an unsuitable colour, for example, a fir tree that turns yellow when it flowers. If a suitable background cannot be provided, an artificial one would need to be provided.
The distant signal shows a yellow lamp in the caution position and a green lamp in the clear position.
Norway
- Next Home signal clear.
- Y = Yellow (unlit)
- FG = Flashing Green (lit)
Home & Distant Signal
Where signals are closely spaced, home signals and distant signals can be mounted on the same post. The two signals are interlocked or "slotted" so that the distant signal can only clear if the home signal is clear.
With mechanical signals, two lights are always shown.
With colour light signals, only one light is shown.
See Also
References
