[Yes, I only just saw this thread.]

In message , Steve Fitzgerald
] wrote:
What seems to be rare on LU is three aspect signals. There used to be one
at the entrance to the Southbound platform at East Putney but that might
be BR practice.

On LU They are generally speed controlled signals and work in a subtly
different way although they are still stop signals and have a train
stop.

If they show green, there is a clear route set and no speed checking is
in place.

If they show red, speed checking is in place and the signal will show a
yellow aspect and the train stop will drop when the train's speed has
been proved below a set figure.

I'm not sure if any still do, but they used to show red+yellow (with
train stop up) to show the train had entered the timing track circuit,
then change to plain yellow (with train stop down) after sufficient time
to ensure the train had slowed enough (if it hadn't, it would be tripped
before the signal could change).

Of course where we share with BR then their signalling practices are in
force.

Only when it's their track.

There *are* true LU three- and four-aspect signals on the Metropolitan
Line north of Harrow-on-the-Hill.

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