Single track signalling.

[norbitonflyer]

I was waiting at a station on a single track line this afternoon and noticed that after a train had passed in one direction the signal went to red behind it as expecred, but then cleared to yellow, despite the next train being scheduled, and duly turning up, from the direction supposedly protected by the signal.
Thus I could see a train approaching on a single track despite there being a signal showing a proceed aspect, for the opposite direction, for the section occupied by that train.
Is this normal?
(The station was Rogerstone on the Ebbw Vale line)

 

[brad465]

Did it definitely go red first? Single lines/2 aspect signal areas are known for having distant signals that show yellow as a minimum aspect, but it doesn't separate signal blocks, so another red signal will be protecting it.

 

[Railsigns]

The signal at Rogerstone station (PJ1944) is a two-aspect distant signal, capable of displaying only yellow or green.

 

[norbitonflyer]

The signal at Rogerstone station (PJ1944) is a two-aspect distant signal, capable of displaying only yellow or green.

Ah, that explains it. I must have imagined/assumed it went to red first

 

[David Dunning]

There is a signal on the Windermere line, at Staveley station, that I have never seen display anything other than a green aspect. It doesn’t even change when a Kendal bound train goes past.

 

[Annetts key]

Okay, there is no simple across the board answer.

I can’t speak about the specific location mentioned. But rather general principles.

If a line is not expected to have an intensive service, then a lower cost signal scheme may be used. Two aspect signalling is such a scheme. Under this arrangement there can be surprisingly large distances between ‘stop’ signals. Each ‘stop’ signal protects the section of line between it and the next ‘stop’ signal.

Each ‘stop’ signal will have only red and green aspects. At service breaking distance from the ‘stop’ signal, on the approach to it, there will be a ‘colour light repeater’ or distant. These can only show yellow (‘stop’ signal ahead is red) or green (signal ahead is also green). As these only give the driver a warning of the state of the signal ahead, no need to spend money on providing a red aspect.

On the other questions, keep in mind that automatic colour light signals are only normally controlled by track circuits or axle counters. So if a train is travelling in the opposite direction, such a signal will step ‘backwards’ through the sequence (green -> yellow -> red) as the train gets nearer to it. Then as the train passes it (depending on the length of the train), it will then step back to green. Unless it was specified at the time of design to be held at red by additional circuitry, or via a emergency replacement switch (or equivalent) operated by the signaller.

Occasionally, you may encounter a signal that does not form part of the aspect sequence, because it’s not a section signal. These may be provided in case of needing a signal that can be replaced to red in an emergency. Such signals may act like distance signals, but can show red if the emergency control is operated. Or they may show green all the time that trains are running, and only show a red if the emergency control is operated.

They don’t affect the safety of the signalling system, as the ‘stop’ / section signal in rear will protect any train in the section that the signal protects. Having an extra signal in the ‘middle’ does not make any difference.

 

[Inversnecky]

So if a train is travelling in the opposite direction, such a signal will step ‘backwards’ through the sequence (green -> yellow -> red) as the train gets nearer to it. Then as the train passes it (depending on the length of the train), it will then step back to green. Unless it was specified at the time of design to be held at red by additional circuitry, or via an emergency replacement switch (or equivalent) operated by the signaller.

Given this has a train needing to pass red as a matter of course, I’d imagine there must be something about this type of working in the Rule Book?

 

[plugwash]

Since they are passing the signal from behind, I wouldn't expect them to see it until they had already passed it.

 

[D6130]

There is a signal on the Windermere line, at Staveley station, that I have never seen display anything other than a green aspect. It doesn’t even change when a Kendal bound train goes past.

Just had a look at Open Time Trains and the signal at Staveley is not shown. Is it the distant signal for a level crossing? The only reason I can think of for a colour light signal to remain permanently green is if it is operated by a trip wire where the railway passes the end of an airport runway, such as at Valley in Anglesey. Other similar emergency systems - such as at Gatwick, Prestwick, Teesside and Edinburgh airports are nowadays tied in with the normal signalling system. However, there is clearly no airport or airfield at Staveley.

Given this has a train needing to pass red as a matter of course, I’d imagine there must be something about this type of working in the Rule Book?

No....because the driver of a train travelling in the opposite direction will not see those signal aspects.

 

[Inversnecky]

Since they are passing the signal from behind, I wouldn't expect them to see it until they had already passed it.

Yes of course!

 

[Railsigns]

There is a signal on the Windermere line, at Staveley station, that I have never seen display anything other than a green aspect. It doesn’t even change when a Kendal bound train goes past.

It's a stop signal controlled by an emergency replacement switch in Burneside gate box. It was originally provided in May 1973 to satisfy the requirement for automatic half barrier (AHB) level crossings to have protecting signals within ten minutes' running time. Staveley level crossing was at that time an AHB. It's an ABCL now, but the signal remains. It used to have a colour light distant signal, since replaced by a reflectorised distant board.

 

[plugwash]

Staveley level crossing was at that time an AHB. It's an ABCL now,

ABCL, seems to be a pretty minor variation on AHB, so presumablly similar principles would apply.

 

[Railsigns]

ABCL, seems to be a pretty minor variation on AHB, so presumablly similar principles would apply.

An ABCL is fundamentally different from an AHB in that it's locally monitored by train drivers. The ten minute rule doesn't apply.

 

[Lucy1501]

The only reason I can think of for a colour light signal to remain permanently green is if it is operated by a trip wire where the railway passes the end of an airport runway, such as at Valley in Anglesey.

The emergency signals at valley (X and Y) are normally unlit, and only have yellow/red aspects.

 

[Signal Head]

Okay, there is no simple across the board answer.

I can’t speak about the specific location mentioned. But rather general principles.

If a line is not expected to have an intensive service, then a lower cost signal scheme may be used. Two aspect signalling is such a scheme. Under this arrangement there can be surprisingly large distances between ‘stop’ signals. Each ‘stop’ signal protects the section of line between it and the next ‘stop’ signal.

Each ‘stop’ signal will have only red and green aspects. At service breaking distance from the ‘stop’ signal, on the approach to it, there will be a ‘colour light repeater’ or distant. These can only show yellow (‘stop’ signal ahead is red) or green (signal ahead is also green). As these only give the driver a warning of the state of the signal ahead, no need to spend money on providing a red aspect.

On the other questions, keep in mind that automatic colour light signals are only normally controlled by track circuits or axle counters. So if a train is travelling in the opposite direction, such a signal will step ‘backwards’ through the sequence (green -> yellow -> red) as the train gets nearer to it. Then as the train passes it (depending on the length of the train), it will then step back to green. Unless it was specified at the time of design to be held at red by additional circuitry, or via a emergency replacement switch (or equivalent) operated by the signaller.

Occasionally, you may encounter a signal that does not form part of the aspect sequence, because it’s not a section signal. These may be provided in case of needing a signal that can be replaced to red in an emergency. Such signals may act like distance signals, but can show red if the emergency control is operated. Or they may show green all the time that trains are running, and only show a red if the emergency control is operated.

They don’t affect the safety of the signalling system, as the ‘stop’ / section signal in rear will protect any train in the section that the signal protects. Having an extra signal in the ‘middle’ does not make any difference.

The usual practice I've seen (going back over many years) has been for the autos on bidirectional sections to be kept at red until a route is set into section, when the relevant ones are allowed to clear, so the step-down you mention wouldn't happen. I've seen it happen on unidirectional lines when working banghrosd, eg SLW during engineering works.

I believe this requirement was removed from Standards a while back, but on the last job I was involved with which provided such signals, the control to red practice was applied anyway, as it was the approach preferred by all the engineers involved.

 

[Railsigns]

I believe this requirement was removed from Standards a while back, but on the last job I was involved with which provided such signals, the control to red practice was applied anyway, as it was the approach preferred by all the engineers involved.

I recall that it happened the other way around. The full bi-directional signalling on the Down Berwick line between Grantshouse and Innerwick is an installation from 1987 where the auto signals can (still) be observed stepping down through the aspects as a train approaches from behind, just predating the requirement for such signals to be controlled to red. I have no recollection of that requirement later being removed from the standards. It's a moot point now anyway, since automatic signals are no longer being provided in new schemes.

 

[MadMac]

I recall that it happened the other way around. The full bi-directional signalling on the Down Berwick line between Grantshouse and Innerwick is an installation from 1987 where the auto signals can (still) be observed stepping down through the aspects as a train approaches from behind, just predating the requirement for such signals to be controlled to red. I have no recollection of that requirement later being removed from the standards. It's a moot point now anyway, since automatic signals are no longer being provided in new schemes.

As @Railsigns is aware, I was the designer on the Grantshouse job. The question of this did come up at the time: the decision was to follow the principles used on the Ayrshire resignalling where autos “step down“ as a train approaches them. To make them replace to Red for wrong-direction moves would have entailed significant materials and design costs.

 

[Annetts key]

As @Railsigns is aware, I was the designer on the Grantshouse job. The question of this did come up at the time: the decision was to follow the principles used on the Ayrshire resignalling where autos “step down“ as a train approaches them. To make them replace to Red for wrong-direction moves would have entailed significant materials and design costs.

Yes, I’ve encountered three different variations:

• automatic signals have no extra controls (not even an emergency replacement facility) so work as I described above,
• automatic signals have no extra controls, but when trains travel in the opposite direction, the signaller holds them at red by using the emergency replacement facility,
• automatic signals have extra controls so will only clear up if a train has been routed into the section in the direction to which they apply.

Part of what is decided (past tense) depends on the amount of money available and the preferences of operations when specifying what they want.

 

[edwin_m]

With relay-based signalling, making the wrong-direction signals remain at red would involve significant extra hardware, because aspects are determined by relays in trackside cabinets. SSI and later computer-based interlockings determine the aspect in the central interlocking, which involves some extra geographic data but no more hardware, so represents much less extra cost. Similar reasoning helps explain the move to having replacements on all auto signals, and probably contributed to the eventual abandonment of autos completely.

 

[Annetts key]

With relay-based signalling, making the wrong-direction signals remain at red would involve significant extra hardware, because aspects are determined by relays in trackside cabinets. SSI and later computer-based interlockings determine the aspect in the central interlocking, which involves some extra geographic data but no more hardware, so represents much less extra cost. Similar reasoning helps explain the move to having replacements on all auto signals, and probably contributed to the eventual abandonment of autos completely.

Except that you do get automatic signals on some SSI / computer based interlocking schemes. However, they all have emergency replacement facilities available to the signaller. I don’t know if new scheme will have automatic signals.

The biggest difference is that new or altered signals that work automatically are no longer provided with automatic signs. As the rule book clause that enabled a driver to pass an automatic signal at danger under their own authority has been removed.

Also of interest, in some situations in the past, some automatic signals were deliberately not provided with automatic signs because they ‘protected’ things like tunnels or other special circumstances.

 

[Railsigns]

Except that you do get automatic signals on some SSI / computer based interlocking schemes. However, they all have emergency replacement facilities available to the signaller.

There are some automatic signals in early SSI areas that can't be replaced to danger by the signaller.

I don’t know if new scheme will have automatic signals.

I covered this at the end of post #16. They won't.