Associated Position Light signal movements

[Jessey]

Hi,

Just looking for some clarification regarding permissive movements.

Scenario

A signal displays an associated position light (APL) with a junction indicator for a permissive movement onto a diverging line.

Beyond the signal and prior to the cross-over points there is a ground position light (GPL) signal.

My query is how far is the initial movement authorised?

Ofcourse I would obey the GPL and never assume it's clear and processed at caution stopping short or any obstruction etc.

However, given that the initial signal has an APL and a Junction Indicator - I would assume thats telling us the route for which the movement has been set so ordinarily the following GPL would be OFF?

What are your views on this - any input would be appreciated.

Thanks

 

[Tomnick]

From the driver’s point of view, the authority for any subsidiary signal cleared is to proceed only as far as the next shunt signal (or to an obstruction of course).

In practice, a calling-on route with a platform number will invariably take you right into the platform and any intermediate shunt signals will invariably be ‘preset’ - but never assume...

 

[MadMac]

Welcome aboard! Signal Engineer of some 41 years here. Could you enlighten me as to what you mean by an APL?

 

[Jessey]

From the driver’s point of view, the authority for any subsidiary signal cleared is to proceed only as far as the next shunt signal (or to an obstruction of course).

In practice, a calling-on route with a platform number will invariably take you right into the platform and any intermediate shunt signals will invariably be ‘preset’ - but never assume...

Thanks for your reply much appreciated.
Fully agree with that

Welcome aboard! Signal Engineer of some 41 years here. Could you enlighten me as to what you mean by an APL?

Sure the abbreviation APL stands for a Associated Position Light signal which is fixed to a main aspect signal

 

[MadMac]

Ah! New one on me, I must confess. Always called them "subsidiary" or "sub". In any event, Tomnick has summed it up.

 

[moggie]

I would say that the MA is to the rear of the train standing in section.

In the scenario described it is presumed that the designated permissive section lies beyond the crossover and that is where the driver is expecting to encounter the train already in section. The route from the APL will be a Call On class route. The interlocking will only clear the APL providing the appropriate section of line IS OCCUPIED and all other controls satisfied, otherwise the APL for the route cannot display a proceed aspect. 'Other controls' will include the GPL in route at the crossover displaying an OFF aspect. This signal will be automatically 'pre-set' by the interlocking (a state where the interlocking commands the GPL to clear to OFF aspect providing its signal aspect controls are satisfied - and hence why as stated in the OP the aspect should be observed and obeyed) as part of the APL route setting. The exit of the Call On route will be the signal designated as the exit signal BUT with the proviso that the entrance signal is being cleared onto an occupied section as indicated by the APL + Indicator.

 

[MarkyT]

For the call-on route from the home signal APL into the occupied platform, there should be a route indication to discriminate it from any shunt route that exists from the same APL only as far as an intermediate GPL.

 

[ComUtoR]

Could the Ground Position light be 'running' ?

 

[Jessey]

I would say that the MA is to the rear of the train standing in section.

In the scenario described it is presumed that the designated permissive section lies beyond the crossover and that is where the driver is expecting to encounter the train already in section. The route from the APL will be a Call On class route. The interlocking will only clear the APL providing the appropriate section of line IS OCCUPIED and all other controls satisfied, otherwise the APL for the route cannot display a proceed aspect. 'Other controls' will include the GPL in route at the crossover displaying an OFF aspect. This signal will be automatically 'pre-set' by the interlocking (a state where the interlocking commands the GPL to clear to OFF aspect providing its signal aspect controls are satisfied - and hence why as stated in the OP the aspect should be observed and obeyed) as part of the APL route setting. The exit of the Call On route will be the signal designated as the exit signal BUT with the proviso that the entrance signal is being cleared onto an occupied section as indicated by the APL + Indicator.

For the call-on route from the home signal APL into the occupied platform, there should be a route indication to discriminate it from any shunt route that exists from the same APL only as far as an intermediate GPL.

Thank you both for your replies they are very informative and I've learnt a lot from this thread already.

Definitely makes senses in terms of differentiating the permissive movements for both lines.

Naturally the APL on it's own would authorise a movement as far as the GPL signal.

My thought process for the APL with a Junction Indicator was that it's telling us that the occupied section is on the diverging line and as a result the GPL would ordinarily be OFF - however, observed and obeyed.

I guess from a signalling perspective I would think that an APL with a Junction Indicator could not be displayed unless the GPL is OFF

Otherwise if you did stop at the GPL and it subsequently cleared you would only be sure the correct route has been set by visually checking the points or via calling the signaller to confirm it.

 

[MarkyT]

Could the Ground Position light be 'running' ?

In modern route setting interlocking, that term isn't really applicable. A 'running' intermediate shunt is set prior to a main or call on route reading over it to the same destination. This has a particular advantage in mechanical interlocking as it can reduce the amount of separate mechanical locking required for the oversetting route lever, instead simply requiring the intermediate shunt lever operated to lock that part of the longer route.

 

[edwin_m]

The modern equivalent is presumably the preset shunt, which clears automatically when a main route is set through it but can be replaced in an emergency. I seem to recall there were pages and pages in the SSI handbook on how to prepare the data for it!

 

[MarkyT]

I guess from a signalling perspective I would think that an APL with a Junction Indicator could not be displayed unless the GPL is OFF
Otherwise if you did stop at the GPL and it subsequently cleared you would only be sure the correct route has been set by visually checking the points or via calling the signaller to confirm it.

Correct, the GPL is proved cleared before the APL and associated route indication is allowed to clear.
Although provided in early colour light schemes, a Junction Indicator is no longer permitted to be used for new installations with a position light aspect, so a separate alphanumeric indicator must be used. That will normally be a different such indicator to the one displayed for the main aspect route to the same destination, to reduce the possibility of driver error (where an alphanumeric is also used for the main aspect). Ref: Rail Industry Standard RIS-0758-CCS Issue: 1.1 Date: March 2018, section 2.6 - https://catalogues.rssb.co.uk/rgs/standards/RIS-0758-CCS Iss 1.1.pdf

The modern equivalent is presumably the preset shunt, which clears automatically when a main route is set through it but can be replaced in an emergency. I seem to recall there were pages and pages in the SSI handbook on how to prepare the data for it!

The disadvantage of the old 'running shunt' arrangement in mechanical areas is that the intermediate GPL lever is locked in the off position so can't be replaced in an emergency until the oversetting main/call-on lever is first replaced.

 

[Jessey]

Correct, the GPL is proved cleared before the APL and associated route indication is allowed to clear.
Although provided in early colour light schemes, a Junction Indicator is no longer permitted to be used for new installations with a position light aspect, so a separate alphanumeric indicator must be used. That will normally be a different such indicator to the one displayed for the main aspect route to the same destination, to reduce the possibility of driver error (where an alphanumeric is also used for the main aspect). Ref: Rail Industry Standard RIS-0758-CCS Issue: 1.1 Date: March 2018, section 2.6 - https://catalogues.rssb.co.uk/rgs/standards/RIS-0758-CCS Iss 1.1.pdf
View attachment 79324

The disadvantage of the old 'running shunt' arrangement in mechanical areas is that the intermediate GPL lever is locked in the off position so can't be replaced in an emergency until the oversetting main/call-on lever is first replaced.

Thanks for clarifying and the reference I'll be sure to take a look at it.

 

[Jessey]

Correct, the GPL is proved cleared before the APL and associated route indication is allowed to clear.
Although provided in early colour light schemes, a Junction Indicator is no longer permitted to be used for new installations with a position light aspect, so a separate alphanumeric indicator must be used. That will normally be a different such indicator to the one displayed for the main aspect route to the same destination, to reduce the possibility of driver error (where an alphanumeric is also used for the main aspect). Ref: Rail Industry Standard RIS-0758-CCS Issue: 1.1 Date: March 2018, section 2.6 - https://catalogues.rssb.co.uk/rgs/standards/RIS-0758-CCS Iss 1.1.pdf
View attachment 79324

The disadvantage of the old 'running shunt' arrangement in mechanical areas is that the intermediate GPL lever is locked in the off position so can't be replaced in an emergency until the oversetting main/call-on lever is first replaced.

The modern equivalent is presumably the preset shunt, which clears automatically when a main route is set through it but can be replaced in an emergency. I seem to recall there were pages and pages in the SSI handbook on how to prepare the data for it!

Thanks, I shall take a look at this also

I think I'll be reading up on the principles of signalling as from a drivers perspective we don't go into much depth as to how routes are proved from a technical aspect.
Although this isn't particularly necessary for the role it would serve in providing a better understanding which would be beneficial.

 

[TheEdge]

Worked example for the OP from the approaches into Norwich.

To allow shorter shunt moves in Norwich station throat there is a bank of running position lights, one for each line, each with a route indicator. CO531, the last main aspect before the station approaching from the coast, can display 9 different routes from its theater box when its APL is cleared. It can only display 6 of those if the main aspect is off. So if I approached it and received the APL with a route indication of 1 - 6 I know that I am routed into a platform and its very likely the running positions lights are cleared, however as the move is authorised by an APL I'm driving under caution and am expecting those running lights to be on. (In practice there are also some non standard OFF indicators under Carrow Road bridge but we shall ignore those here). However I do know that the running positions light should be showing the same route as CO531.

However if I approach it and receive a E, C or W at CO531 I know right away I am going into a siding and there is a significantly higher chance that the running position lights will be on. They will also be displaying a totally different route to CO531 (1-6, J, M, Y or S). So once again I'm driving at caution as the move was authorised by an APL.

So there is only one common thought between those two approaches. Despite the indications from CO531 I am ALWAYS driving onwards at caution because the move has been authorised by an APL so is only as far as the running position lights in the throat under caution. If anyone had a SPAD and their defense was CO531 was showing a platform route would have no leg to stand on.

 

[LAX54]

Worked example for the OP from the approaches into Norwich.

To allow shorter shunt moves in Norwich station throat there is a bank of running position lights, one for each line, each with a route indicator. CO531, the last main aspect before the station approaching from the coast, can display 9 different routes from its theater box when its APL is cleared. It can only display 6 of those if the main aspect is off. So if I approached it and received the APL with a route indication of 1 - 6 I know that I am routed into a platform and its very likely the running positions lights are cleared, however as the move is authorised by an APL I'm driving under caution and am expecting those running lights to be on. (In practice there are also some non standard OFF indicators under Carrow Road bridge but we shall ignore those here). However I do know that the running positions light should be showing the same route as CO531.

However if I approach it and receive a E, C or W at CO531 I know right away I am going into a siding and there is a significantly higher chance that the running position lights will be on. They will also be displaying a totally different route to CO531 (1-6, J, M, Y or S). So once again I'm driving at caution as the move was authorised by an APL.

So there is only one common thought between those two approaches. Despite the indications from CO531 I am ALWAYS driving onwards at caution because the move has been authorised by an APL so is only as far as the running position lights in the throat under caution. If anyone had a SPAD and their defense was CO531 was showing a platform route would have no leg to stand on.

Indeedy CO531 to say P6 occupied, will be one signalled route as far as the box is concerned, CO531 will be a two whites with 6, and the GPL in the middle of the route will show two whites and a 6. If P6 was clear, then CO531 would be SY and '6' with the GPL showing two whites and a 6.

If the route was set from GPL to GPL, (sometimes in error by a learner) then the route would be set from the 'middle' GPL to the destination, say P6 with two whites, then with a route set from the Main signal to the GPL, it would clear to two whites and either E C or W depending on which line the route was set. (this would be regardless of P6 having a train in it, or empty)

When setting routes from GPL's to GPL's you always set the furthest one away first, and ensure it clears, before setting a route up to it. (avoids any chance of a SPAD then)

 

[alxndr]

I think I'll be reading up on the principles of signalling as from a drivers perspective we don't go into much depth as to how routes are proved from a technical aspect.
Although this isn't particularly necessary for the role it would serve in providing a better understanding which would be beneficial.

The thing to be careful with if you do this is to remember that any write up of signalling will explain how it should work in general. The last thing you want to do is end up making assumptions, even subconsciously, and miss what is actually there due to a fault or something that is a little different to the norm. I've seen some strange things in fault situations. Ultimately you need to drive to what is in front of you, not second guess the interlocking. I'm not saying you'd deliberately do this, but the subconscious is a funny thing.

 

[LowLevel]

Being a bit more complex but degraded working... if you're talking a driver past a main aspect at danger using a route that runs though what would generally be a preset/running/route proving GPL/shunt disc/whatever the appropriate name for your given circumstance is...

Would the signaller authorise the driver to pass the main aspect and the GPL(s) in one go or would they expect the driver to stop at the GPL for further instructions (bearing in mind I assume many historically wouldn't have had a telephone fitted).

I ask because a very diligent driver colleague got caught out in exactly that scenario when a signaller called him by the main aspect into a platform but forgot about the GPL between the signal and the platform. I believe it was treated as a cat A SPAD.

I sign a mechanical box with route proving shunt discs fitted and I know how I would handle that but it's not quite the same as a fairly major mainline station throat worked off a traditional NX panel so how it *should* be done interests me.

 

[Tomnick]

Being a bit more complex but degraded working... if you're talking a driver past a main aspect at danger using a route that runs though what would generally be a preset/running/route proving GPL/shunt disc/whatever the appropriate name for your given circumstance is...

Would the signaller authorise the driver to pass the main aspect and the GPL(s) in one go or would they expect the driver to stop at the GPL for further instructions (bearing in mind I assume many historically wouldn't have had a telephone fitted).

I ask because a very diligent driver colleague got caught out in exactly that scenario when a signaller called him by the main aspect into a platform but forgot about the GPL between the signal and the platform. I believe it was treated as a cat A SPAD.

I sign a mechanical box with route proving shunt discs fitted and I know how I would handle that but it's not quite the same as a fairly major mainline station throat worked off a traditional NX panel so how it *should* be done interests me.

There's no reason why the driver shouldn't be authorised to pass the GPL(s) in the same conversation as the authority to pass the main signal (S5 1.3 prohibits authority being given to pass two main signals at once on a TCB line but that's all) and I'd certainly prefer it that way (from both sides!). It'd take forever to get into Sheffield otherwise, especially as you suggest if for whatever reason GSM-R wasn't available! I'd go so far as to say that the same could apply at a location on an AB line where it's necessary to talk past two or more stop signals at once...

 

[EvoIV]

The thing to be careful with if you do this is to remember that any write up of signalling will explain how it should work in general. The last thing you want to do is end up making assumptions, even subconsciously, and miss what is actually there due to a fault or something that is a little different to the norm. I've seen some strange things in fault situations. Ultimately you need to drive to what is in front of you, not second guess the interlocking. I'm not saying you'd deliberately do this, but the subconscious is a funny thing.

This sums it up perfectly for me. As a driver I'm interested in how the signalling works around me but I try, so far as possible, to compartmentalize this so that I just focus on driving to the signals.

I might listen to the subtle hints that certain sequences give you as to what may be happening but I still just dumbly drive to the signals, 99 times out of a hundred I'll allow myself a wry grin as I was right, that one time in 100 I'm sure glad I just dumbly drove to the signals.

As an example, and I'm not sure that this is even physically possible but assume that it is, for the route originally described. You pass the first sub signal with the route indication. Between passing that and getting to the next GPL, there's a power failure/telecoms failure/track circuit blip/urgent call to the signaller/whatever and that signal either goes back or is put back to danger. You, assuming it was going to be off, pass it by twenty foot. Have you had a spad? Yes. Do you have any excuse? No...

No, because the definition of proceeding at caution meant you should have been able to stop. No, because you ignored the meaning of a subsidiary signal. You *might* be able to argue it away on a technicality but I reckon you'd likely have to take the spad. Thus it's really just easier to take the sub signal and straight away start looking for the next one while checking the line ahead is clear. Once you see it's off you can happily carry on with your day.

 

[EvoIV]

On another related but differen't example, there's a route where I need to take the slowest diverging route. Because of this, the route is approach controlled. More often than not the route is clear through but you're still coming up to a red before it clears.

After the double yellow is an AWS magnet indication for the warning board for the diverging speed. This is suppressed if the route isn't set (which conflicts with many other places when you'll get the magnet for every diverging possibility if the route isn't set yet). Thus you'll only get the magnet if the route is set for position 5, which is what I need. Month after month I begin to form this impression but still I'm assuming the red will not come off. Every time I fail to get the magnet the signal stays red. Every time I get the magnet the signal steps up.

After maybe six months, it finally happens. I get the magnet and the signal stays red. It actually made my stomach go over because although I stopped fine I feel like another couple of weeks and I'd have begun making that dangerous assumption and maybe I wouldn't have been able to stop in time? This is a real example which I hope someone can draw something from.

It certainly made me very rigid in my approach to this kind of thing now.

 

[edwin_m]

As an example, and I'm not sure that this is even physically possible but assume that it is, for the route originally described. You pass the first sub signal with the route indication. Between passing that and getting to the next GPL, there's a power failure/telecoms failure/track circuit blip/urgent call to the signaller/whatever and that signal either goes back or is put back to danger. You, assuming it was going to be off, pass it by twenty foot. Have you had a spad? Yes. Do you have any excuse? No...

No, because the definition of proceeding at caution meant you should have been able to stop. No, because you ignored the meaning of a subsidiary signal. You *might* be able to argue it away on a technicality but I reckon you'd likely have to take the spad. Thus it's really just easier to take the sub signal and straight away start looking for the next one while checking the line ahead is clear. Once you see it's off you can happily carry on with your day.

Exception being if the GPL drops back to danger when you are too close to stop at it.

Something similar can also happen on a main aspect route with preset shunts. The GPLs will still clear in the normal course of events but could replace due to a failure or emergency. The difference is you aren't driving at caution so can't be expected to stop at the signal.

On another related but differen't example, there's a route where I need to take the slowest diverging route. Because of this, the route is approach controlled. More often than not the route is clear through but you're still coming up to a red before it clears.

After the double yellow is an AWS magnet indication for the warning board for the diverging speed. This is suppressed if the route isn't set (which conflicts with many other places when you'll get the magnet for every diverging possibility if the route isn't set yet). Thus you'll only get the magnet if the route is set for position 5, which is what I need. Month after month I begin to form this impression but still I'm assuming the red will not come off. Every time I fail to get the magnet the signal stays red. Every time I get the magnet the signal steps up.

After maybe six months, it finally happens. I get the magnet and the signal stays red. It actually made my stomach go over because although I stopped fine I feel like another couple of weeks and I'd have begun making that dangerous assumption and maybe I wouldn't have been able to stop in time? This is a real example which I hope someone can draw something from.

Seems to me there is a hazard here your TOC should be raising with NR. I'm rather out of date on these things but I was under the impression warning boards for diverging speed restrictions weren't used when there was an approach control on the route in question, because obeying the aspects would bring the train down to a safe speed.

I think one reason for the magnet being energised and the signal staying at danger would be if the signaller had cancelled the route in the meantime. If the signal doesn't clear, approach locking doesn't come into effect and the signaller can set a conflicting route immediately after cancelling.

 

[Tomnick]

Seems to me there is a hazard here your TOC should be raising with NR. I'm rather out of date on these things but I was under the impression warning boards for diverging speed restrictions weren't used when there was an approach control on the route in question, because obeying the aspects would bring the train down to a safe speed.

I think one reason for the magnet being energised and the signal staying at danger would be if the signaller had cancelled the route in the meantime. If the signal doesn't clear, approach locking doesn't come into effect and the signaller can set a conflicting route immediately after cancelling.

We’ve got a few examples on our routes - they seem increasingly prevalent on new schemes. One example relates to a reduction in permissible speed on the ‘straight’ route beyond the junction (90 reducing to 70 at the junction for all routes, reducing further to 60 for the straight route beyond the junction, so no approach control). All the others that I can think of relate to diverging routes with flashing yellows. In all cases, the magnet is suppressed if a route is set for a direction to which the warning board isn’t applicable (or words to that effect). The distinction between “route set” and “signal cleared” is presumably the critical one here!

 

[alxndr]

I think one reason for the magnet being energised and the signal staying at danger would be if the signaller had cancelled the route in the meantime. If the signal doesn't clear, approach locking doesn't come into effect and the signaller can set a conflicting route immediately after cancelling.

Or the suppressor was simply faulty. It’s impossible to say without guesswork, so all you can do is use your own eyes and ears and treat it all as worst case scenario - i.e. that red signal will stay red.

 

[edwin_m]

We’ve got a few examples on our routes - they seem increasingly prevalent on new schemes. One example relates to a reduction in permissible speed on the ‘straight’ route beyond the junction (90 reducing to 70 at the junction for all routes, reducing further to 60 for the straight route beyond the junction, so no approach control). All the others that I can think of relate to diverging routes with flashing yellows. In all cases, the magnet is suppressed if a route is set for a direction to which the warning board isn’t applicable (or words to that effect). The distinction between “route set” and “signal cleared” is presumably the critical one here!

If there's no approach control from red (with or without flashing yellows) then the hazard doesn't arise because the signal will be clear for the diverging route, or it will be at danger because no route is set, with no expectation that it will clear on approach.

 

[Tomnick]

If there's no approach control from red (with or without flashing yellows) then the hazard doesn't arise because the signal will be clear for the diverging route, or it will be at danger because no route is set, with no expectation that it will clear on approach.

The warning board is something like 600 yards back from the junction signal, so the risk here is that a driver who’s had a single yellow at the previous signal (not uncommon as it’s a busy junction) finds the magnet for the warning board suppressed and assumes that the junction signal must have cleared.

 

[Jessey]

This sums it up perfectly for me. As a driver I'm interested in how the signalling works around me but I try, so far as possible, to compartmentalize this so that I just focus on driving to the signals.

I might listen to the subtle hints that certain sequences give you as to what may be happening but I still just dumbly drive to the signals, 99 times out of a hundred I'll allow myself a wry grin as I was right, that one time in 100 I'm sure glad I just dumbly drove to the signals.

As an example, and I'm not sure that this is even physically possible but assume that it is, for the route originally described. You pass the first sub signal with the route indication. Between passing that and getting to the next GPL, there's a power failure/telecoms failure/track circuit blip/urgent call to the signaller/whatever and that signal either goes back or is put back to danger. You, assuming it was going to be off, pass it by twenty foot. Have you had a spad? Yes. Do you have any excuse? No...

No, because the definition of proceeding at caution meant you should have been able to stop. No, because you ignored the meaning of a subsidiary signal. You *might* be able to argue it away on a technicality but I reckon you'd likely have to take the spad. Thus it's really just easier to take the sub signal and straight away start looking for the next one while checking the line ahead is clear. Once you see it's off you can happily carry on with your day.

Fully agree with the points raised here but my original question was solely relating to the principles of signalling and not in relation to driving techniques.

I did state what my approach would be in such a scenario.

Any such movement should always be made at caution whilst being prepared to stop short of any obstruction etc. Making assumptions as rightly said in this thread is a recipe for disaster.

However, this is an old system and understanding how it works is advantageous.

It has been confirmed that with the APL and JI the following GPL should be OFF - Unless the signaller reverts it or there is some form of failure. In such an event you'd stop and report this to the signaller.


Consider this scenario -
There is some form of equipment failure which causes the GPL to go ON and as you approach it it goes OFF - Stranger things have been known to happen.

A lack of understanding in such a scenario could see that as a normal sequence without realising what could have happened.

Therefore the knowledge reaffirmed in this thread would help prevent a mistake from arising in the said scenario.

Thanks to everyone who contributed and for providing clarification to the initial question.

 

[EvoIV]

Yeah I wasn't meaning to imply you'd fall into the trap, just make the point that experience had told me to try to forget how the signalling works while actually driving.

With regards to my example above, the warning board applies to positions 5 and 6 (though I don't sign the route from position 5) whereas I must have position 6. Position 5 has flashers and a fifty diverging speed as per the warning board. Position 6 goes the same route initially, over the fifty crossover, before further diverging over a 40mph junction. Position 6 is approach controlled and at some point the designer decided to energise the magnet, or rather not suppress it, for both routes as the first crossover is the same in both cases.

I think the point above about route set and signal cleared is the key, I guess the route can always be set without "pulling off".