Below is an animated example
of the three different colour light
signalling systems currently in operation on the London Underground.
I have tried to
keep the principle as simple as possible so as not to confuse.
You will notice that I have not made any mention of "overlaps".
Maybe I will delve deeper another time.
** In all three systems the RED aspect tells the driver to STOP. **
The green aspect tells the driver that at least one section ahead is clear.
(A section being between this signal and the next signal).
Although this would appear to give
quite limited information
it is quite safe and efficient enough for the majority of lines.
The yellow aspect tells the driver that only one section ahead is clear.
The green aspect now tells the driver that at least two sections ahead are
This system allows trains of a
higher speed to travel over the same line as slower trains.
It is used on sections of the District line where British Rail trains share the same
The higher speed trains will start braking on a yellow aspect whilst the slower trains
will not need to brake until they observe a red aspect.
A single yellow aspect tells the driver that only one section ahead is clear.
Two yellow aspects tell the driver that only two sections ahead are clear.
The green aspect now tells the driver that at least three sections ahead are
This system offers enough
information to enable trains with differing top speeds
and differing braking patterns to operate safely over the same line.
This system is used on the Metropolitan line between Harrow on the Hill and Amersham.
The A60/62 Electric Multiple Units only require the RED
and yellow aspects
for safe braking distances to be maintained whilst the British Rail trains
require all FOUR
aspects because of both the higher speeds possible
and the need for goods trains to brake earlier due to their weight.
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