In article . com,
TheOneKEA writes
In a 4-aspect signal, there are three relays:
- the GR relay, which controls the switch from green to double yellow
and back
- the HHR relay, which controls the switch from double yellow to single
yellow and back
- the HR relay, which controls the switch to single yellow to red and
back again

Not quite. The HR controls the switch from red to non-red. Once yellow,
the HHR allows it to switch to double yellow, and then the GR to green.

+- not GR -- yellow --+
| |
+- HHR -+--- GR ----- green --+
| |
+- not HHR -+ |
| | |
+- HR ---+- not GR --+------- yellow --+
| |
+ve -+- not HR --------------------- red ---+-- -ve

Under normal circumstances (i.e. an auto signal on a TCB line), the GR
relay will be energised, thus making the signal head show a green
aspect. When a train crosses the overlap into the track circuit
connected to the signal, the circuit is shunted, causing the GR relay
(and additionally, the HHR relay) to drop out and the HR relay to be
energised, thus switching the signal from green to red.
[...]
(hopefully this isn't completely and totally wrong...)

Um, I'm afraid there are errors in there.

Let's consider a simple bit of track:

1 3 5 7 9
|-O |-O |-O |-O |-O
-------+-----A-----+-----B-----+-----C-----+-----D-----+----

Track circuit A connects to 1's HR relay (1HR).
Track circuit B connects to 3HR and 1HHR.
Track circuit C connects to 5HR, 3HHR, and 1DR.
Track circuit D connects to 7HR, 5HHR, and 3DR.

With all of A to D clear, signal 1 will be green. When the train moves
on to track A, signal 1 will drop to red. Moving on to B drops 3 to red
but, when it clears A, signal 1 can raise to single yellow. When it is
on C, 1DR is down but 1HHR and 1HR are up, so it will show double
yellow. Finally, when track circuit C is cleared, 1DR picks up and the
signal goes green.

If, say, there's a set of trailing points between 7 and 9, 7HR can only
pick up if:
- D is clear
- the signaller has set the route from 7 to 9
- the points are proven to be locked in the correct position.

With facing points, the HHR has to include logic to "look ahead" in the
correct direction.

There's something strangely reassuring about reading dense technical
conversations that you don't even remotely understand. It tells me
that
there is someone, somewhere, who knows what they're talking about ...
Clive does - some would say that Clive knows everything about
signalling.

Far from it.

--
Clive D.W. Feather | Home:
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