As we passed the end of the platform at Three Bridges I could see the signal there was at red.
Things I'm curious about are:
- Did we actually pass a signal at danger or would I just have seen it red as my window passed the signal because it would have changed to red when the front of the train passed it and occupied the next bit of track? (I was seated in the second-from-front carriage.)
The controls that determine when a signal that was showing a proceed aspect goes back to red, depend on a number of factors.
Most 'controlled' signals will go back to red when the first set of wheels of the train travel into the track circuit or axle counter section just after (beyond) the signal. This section is called the overlap.
An overlap is normally a requirement for lines signalled for passenger trains. The distance between the signal and the start of this 'overlap' track circuit or axle counter section can be as little as 10cm. Yes, four inches. Or it could be two or three metres (yards).
Either way, by the time that the second coach passes the signal, it's going to be red.
About the only time that this does not apply to a 'controlled' signal, is that when the signalling system was designed, operations specified that they wanted the signal to have special 'last wheel replacement' controls.
These were normally only required if long trains would call at the station and the locomotive would have to stop after (beyond) the signal for station duties. Or where there were (frequent) shunting moves where a train would be propelled by the loco at the rear of the train.
The deliberate intention is for the signal to continue to show a proceed aspect until most or all of the train (depending on its length) has passed the signal.
These days, these special controls are fairly rare.
Outside of areas where controlled signals are used (junctions, protecting level crossings, large busy complex stations) on plain line, you will find 'automatic' signals. These normally work automatically without needing any input from the signaller. Indeed, on older signalling systems, the signaller may have no control of many of this type.
With these, normally there is no separate overlap section. Instead the approach (also called the berth) section and the overlap section are combined into one longer section. The next track circuit / axle counter section will normally commence between 183 metres (200 yards) and 274 metres (300 yards) (approximately) beyond the signal. Only when a train gets to this section will the signal to back to red.
This is long enough for passengers to see the signal showing a proceed aspect from the second coach (or maybe from even further back).
In the past, automatic signals were fitted with an automatic signal plate / sign, but the railway rules have changed and there is no longer a requirement for these plates / signs to be provided (the rule change makes them no longer relevant).