Brighton - London line: Bi-Di signalling query

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aleggatta

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Whilst travelling the A23 tonight heading south towards the junction with the A27) earlier I saw the down line showing a green aspect (normal) but the down signal on the up line showing double yellows. Is it normal for the 'wrong' direction signals to show anything other than red when bi-di is not in use or was this an unusual moment for two down direction moves on two parallel lines? Apologies if this has been covered before, just don't think I've ever seen two clear signals at that location in the same direction before?
 
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4COR

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Whilst travelling the A23 tonight heading south towards the junction with the A27) earlier I saw the down line showing a green aspect (normal) but the down signal on the up line showing double yellows. Is it normal for the 'wrong' direction signals to show anything other than red when bi-di is not in use or was this an unusual moment for two down direction moves on two parallel lines? Apologies if this has been covered before, just don't think I've ever seen two clear signals at that location in the same direction before?
It's the outer distant (RR on plate - you can see it on streetview - it's signal T423RR) I believe - it can only show double yellow or green.
 
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TSG

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I don't know the place but from your question I'd infer that you are used to seeing a red aspect on the 'wrong' direction signal i.e. it is not a distant incapable of displaying a red aspect. For a (non distant) signal to show a proceed aspect the route must be set, and to set a route on a bi directional line the interlocking proves that the opposing route is not set, so that is two parallel moves
 

Sunset route

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Whilst travelling the A23 tonight heading south towards the junction with the A27) earlier I saw the down line showing a green aspect (normal) but the down signal on the up line showing double yellows. Is it normal for the 'wrong' direction signals to show anything other than red when bi-di is not in use or was this an unusual moment for two down direction moves on two parallel lines? Apologies if this has been covered before, just don't think I've ever seen two clear signals at that location in the same direction before?

That would be totally correct, only the new fully bi directional signalling between Copyhold Junction and Balcombe Tunnel Junction hold the wrong direction signals including the distant signals at red until the bi directional signalling is used.
 
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Annetts key

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This is a generic answer, as I don’t know the details of that locality.

The answer to the question, of what a reversible signal can show (or indeed, a normal direction signal when the reversible signalling is in use) depends on the type of interlocking, if the signal is an automatic or a controlled signal, and if it is a stop signal or a distant signal. It also depends on what the specification was when designed. And if the signal has a ‘emergency’ replacement switch or control at the signal box/PSB/panel/signalling centre/control centre/ROC or other fancy named place...

In some areas, automatic reversible signals were provided. These are reversible signals that show an aspect depending on if the sections ahead of them are clear. I once had a report from a driver (running in the normal direction on the adjacent line) saying that he saw the reversible signal step down from a yellow to a red. This driver thought this was odd, as he thought it should be always be red unless the reversible signalling was in use.

What had happened, is at the previous weekend, the reversible signalling had been in use. Normally the signallers held the reversible signal at red via the ‘emergency’ replacement switch that was provided. But obviously during the time that they were using the reversible signalling, this switch was put in the ‘automatic’ position. When they went back to normal working, the switch had been left in the automatic’ position.

The driver saw the reversible signal step down from a yellow to a red, because a train was travelling in the normal direction on the line that the reversible signal applied to. Hence as the track circuit ahead on the line to which the reversible signal applied, went occupied as a train approached, the signal responded as designed.

For controlled signals, they always sit at their most restrictive aspect (red for stop signals) unless the signaller (or Automatic Route Setting - ARS system) calls a valid route for them. And all the conditions are correct (and proved correct by the interlocking), before they are allowed to clear up (to show) a proceed aspect. This will include making sure that there is no opposing route set or called.

For some systems, automatic signals on reversible lines only clear to a proceed aspect if a controlled signal is routed on to the reversible line. Or the controlled exit signal (to exit a reversible section) is routed (it does not have to actually show a proceed aspect). At all other times, the automatic reversible signals show their most restrictive aspects.

Yet another design has other controls to keep the reversible signals at their most restrictive aspects unless the signaller operated his/her control to enable reversible working.
 
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Taunton

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The driver saw the reversible signal step down from a yellow to a red, because a train was travelling in the normal direction on the line that the reversible signal applied to. Hence as the track circuit ahead on the line to which the reversible signal applied, went occupied as a train approached, the signal responded as designed.
This is how Automatic Block Signals work in the USA on single lines - they are fully automatic, no reference to any signaller, and show green in both directions until a train comes along, when they start to step down. They are just a safety feature, the actual control of which trains are to run is controlled bt train orders, or their modern equivalents. It's a useful cue if you can see them that a train is approaching in the opposite direction. Such installations never really implemented in Britain.
 
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