Annn1

I need help here. Four numbers on a signal plate generally meant it was a Draw-up (in my day a Permissive) signal that worked with the semi automatic signal ahead usually controlling a junction. It would not clear to green unless the speed of the train was reduced to a safe breaking distance that the signal ahead at red was protecting.

For automatic signals it may have been some form of speed control.

They can also be used when a signal has been added between two existing ones

I'm wondering if the examples you quote above might be 'Round-The-Benders'. This is slang for additional signals that were installed in curved tunnel sections in the years following the Stratford collision of 1953, with the sole purpose of ensuring that there was always at least one signal capable of displaying a red aspect visible to drivers at all times. They had no purpose with respect to headway as they had the same limit of control as the signal in rear.

Thanks

I was just wondering if I might ask for some clarification. I'm not really sure I follow why, after the Stratford collision, these additional signals were considered necessary. Am I right in thinking the Stratford collision followed from a stop and proceed where the driver passed a signal at danger under rule but failed to control his speed afterwards and collided with a train in front? I thought we got SCAT for that, but I'm not really sure I follow how the additional signals would help, or how they work. Sorry, I wonder if you could just explain what is meant by having the same limit of control as the signal in rear.

Ta!

The 4-digit signals seem to fit into three categories, which I presume depends on date of installation rather than anything else.

1) Basically normal signals, complete with overlaps and repeaters, at which you can come to a stand if there is a train in the section ahead. I think this basically applied to some parts of the Northern Line, certainly I've stopped at A.7171 and A.7172 between South Wimbledon and Morden, both of which also had repeaters.

2) Those without a defined overlap, and which clear at the same time as their associated headway signal, but which had repeaters. There were some around the Kennington to Embankment area, where you could see certain signals go straight from red to green over green. Presumably green over yellow would only ever be seen under failure conditions, and you would never normally stop at the 4-digit signal.

3) Those without a defined overlap, and with no repeater. There was one north of Euston CX, where if the 4-digit signal were 'on', there would be insufficient sighting distance to stop from line speed. I don't recall this ever happening however, I never ascertained if the 4-digit signal has to be proved clear in order for the preceding signal to clear. My understanding is this is certainly the case for the ones on the Bakerloo.