Lee-Hither Green signalling sequence

[brad465]

Today something rare happened (at least to my eyes): the signal at the end of Lee Platform 1 displayed a double-yellow aspect (For the 10:35 Dartford-Charing X). Almost always the aspect for trains routed towards Hither Green is either yellow or green; green provides full clearance through Hither Green, but yellow forces trains to stop just before, where they are then given a yellow or other aspect depending on signal block availability ahead.

Does anyone know what exactly goes on here? My initial thought was TPWS/signal overlap reasons mean a train cannot be routed into Hither Green P5 unless it's clear out the other end, or until the train has been checked down to a reduced speed in a similar manner to junction approach control for diverging routes. However this doesn't explain how double-yellow at Lee was possible today.

 

[zwk500]

Today something rare happened (at least to my eyes): the signal at the end of Lee Platform 1 displayed a double-yellow aspect (For the 10:35 Dartford-Charing X). Almost always the aspect for trains routed towards Hither Green is either yellow or green; green provides full clearance through Hither Green, but yellow forces trains to stop just before, where they are then given a yellow or other aspect depending on signal block availability ahead.
Does anyone know what exactly goes on here? My initial thought was TPWS/signal overlap reasons mean a train cannot be routed into Hither Green P5 unless it's clear out the other end, or until the train has been checked down to a reduced speed in a similar manner to junction approach control for diverging routes. However this doesn't explain how double-yellow at Lee was possible today.

I would guess that the signal at the London end of Hither Green P5 (TL294) has a normal overlap that obstructs the junction, and a short (or Restricted) overlap that doesn't. When a Restricted Overlap is used, the signal in rear (TL342, in this case) will be held at Red until the train approaches before it steps up. This will hold the signal on the London end of Lee P1 (TL344) at single Yellow.

If the route from TL294 is at set forward, it will be displaying a Yellow or better aspect, and TL342 will be displaying double yellow or better, and TL344 Green. However, if the normal overlap was selected but TL294 held at red, this would give TL342 single Yellow and TL344 double yellow. As there would not normally be many reasons to select the normal overlap without setting a forward route, this would explain why you haven't seen double yellow at Lee before. The most obvious circumstances for this would be that the block section ahead of TL294 (TL290, the signal in rear of the signal protecting Parks Bridge Jn/Courthill South Jn (TL286)) is still occupied but the next movement over the junction will be the departure from P5, so the overlap obstructing the junction doesn't matter.

NOTE: I don't have any information explicitly confirming the arrangements, this is an interpretation based of the diagrams I do have access to. There may be a signaller or driver who covers the area who can confirm if my hunch is correct.

 

[TransportHub]

At risk of looking at this too simply, TL344 was at double yellow, so TL342 would be single yellow and TL294 at red protecting the junction. Wouldn't that be the normal expected situation? Or an I missing something?

 

[dodecahedron67]

TL342 looks to be release from Red when a conflicting movement (TL291 to TL295 or TL296 to TL290) is set at Hither Green Station Jn under a restricted overlap like @zwk500 said. I imagine that due to the frequency of trains, the non-restricted overlap (i.e no conflicting route set, but also no route set from TL294) will be used so infrequently that the normal sequence will be a single yellow at the end of the platform of Lee.

 

[zwk500]

At risk of looking at this too simply, TL344 was at double yellow, so TL342 would be single yellow and TL294 at red protecting the junction. Wouldn't that be the normal expected situation? Or an I missing something?View attachment 206994

Your screenshot shows that TL342 at red despite the route being set is the normal situation.

 

[brad465]

I would guess that the signal at the London end of Hither Green P5 (TL294) has a normal overlap that obstructs the junction, and a short (or Restricted) overlap that doesn't. When a Restricted Overlap is used, the signal in rear (TL342, in this case) will be held at Red until the train approaches before it steps up. This will hold the signal on the London end of Lee P1 (TL344) at single Yellow.

If the route from TL294 is at set forward, it will be displaying a Yellow or better aspect, and TL342 will be displaying double yellow or better, and TL344 Green. However, if the normal overlap was selected but TL294 held at red, this would give TL342 single Yellow and TL344 double yellow. As there would not normally be many reasons to select the normal overlap without setting a forward route, this would explain why you haven't seen double yellow at Lee before. The most obvious circumstances for this would be that the block section ahead of TL294 (TL290, the signal in rear of the signal protecting Parks Bridge Jn/Courthill South Jn (TL286)) is still occupied but the next movement over the junction will be the departure from P5, so the overlap obstructing the junction doesn't matter.

NOTE: I don't have any information explicitly confirming the arrangements, this is an interpretation based of the diagrams I do have access to. There may be a signaller or driver who covers the area who can confirm if my hunch is correct.

Thank you for this.

I do wonder if one day it's worth re-siting TL294 further forward slightly and extending P5 back towards Lee to compensate, so the normal overlap doesn't obstruct the junction. Right now it would be an issue as the entrance and cross-platform walkway to P4 would no longer be aligned with where trains call, however the new entrance and accessible footbridge being built further back will address this among the wider benefits this work is bringing. The linespeed would probably have to drop to account for the shorter signal section, however nothing does 60mph through there anyway, as it's either called at Lee or is slowing down to 20mph by this point anyway. 40-50mph between Lee and Hither Green on the up would suffice.

 

[Stephen42]

I would guess that the signal at the London end of Hither Green P5 (TL294) has a normal overlap that obstructs the junction, and a short (or Restricted) overlap that doesn't. When a Restricted Overlap is used, the signal in rear (TL342, in this case) will be held at Red until the train approaches before it steps up. This will hold the signal on the London end of Lee P1 (TL344) at single Yellow.

If the route from TL294 is at set forward, it will be displaying a Yellow or better aspect, and TL342 will be displaying double yellow or better, and TL344 Green. However, if the normal overlap was selected but TL294 held at red, this would give TL342 single Yellow and TL344 double yellow. As there would not normally be many reasons to select the normal overlap without setting a forward route, this would explain why you haven't seen double yellow at Lee before. The most obvious circumstances for this would be that the block section ahead of TL294 (TL290, the signal in rear of the signal protecting Parks Bridge Jn/Courthill South Jn (TL286)) is still occupied but the next movement over the junction will be the departure from P5, so the overlap obstructing the junction doesn't matter.

NOTE: I don't have any information explicitly confirming the arrangements, this is an interpretation based of the diagrams I do have access to. There may be a signaller or driver who covers the area who can confirm if my hunch is correct.

This looks to be the case, screenshot below from 21:41 today. 2F69 would be going slowly with signal 286 at red ahead. The junction was clear with the route able to be reset for platform 5 departure and that's enough overlap at 294 for signal 342 to clear without approach control. 286 is approach controlled towards Lewisham which makes slowly moving trains more likely, usually the train would have passed 290 though.

A train approaching Hither Green (either from Lee or Grove Park on slows where same situation applies) would go a bit faster than the approach controlled sequence. It might happen automatically when the route is called and think that area might not have automatic route setting (ARS) currently.

 

[zwk500]

Thank you for this.

I do wonder if one day it's worth re-siting TL294 further forward slightly and extending P5 back towards Lee to compensate, so the normal overlap doesn't obstruct the junction.

I suspect not because of the impact on signal spacing in what is quite a dense traffic area. A standard overlap is 180m or 183m I think (200yds), although this would be slightly lower in this instance at a 60mph maximum approach speed. A standard overlap from 60mph would still be at least 120m though, and that overlap would need to be at a track circuit boundary/axle counter positioned clear of the fouling point for the junction.

The next significant changes to the position of signals in this area is likely to be when ETCS L2 comes in. That's going to be quite some time away from Hither Green.