Clive D. W. Feather wrote:

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

This is the sort of thing I hoped to see - thanks :-)


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

As I expected :-((


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.

Thanks; I get it now. You should write another article for your site
that explains all of this.


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.

So what does the circuit fiagram for this logic look like? Presumably
it involves the detector boxes on the point blades as well as linkages
to the HHR relays in the signals in advance of the facing points.


Far from it.


Maybe not everything, but certainly enough to seem like it...