Tom Anderson wrote:

I'm curious as to what that sentence even means!

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

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. After the train
has crossed the overlap of the next signal and moved off of the track
circuit, the HR relay drops out, causing a circuit connected to the
next track circuit to energise; this is what lights the single yellow
aspect. After the train moves off of that track circuit onto the next
one, the process is repeated with the HHR relay, causing the second
yellow aspect to be lit. Finally, once three track circuits are clear
between this signal and the train, the HR and HHR relays drop out and
the GR relay is energised, which finally lights the green aspect.

(hopefully this isn't completely and totally wrong...)


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.