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PoSA

You are here: Home > Forum > Miscellaneous > The real thing (signalling) > PoSA

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PoSA 01/05/2015 at 22:43 #71612
John
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How are PoSA signals activated on the panel / VDU?

Why aren't they more widely used?

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PoSA 02/05/2015 at 02:30 #71617
Muzer
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As for the second question, I believe they were only invented recently — correct me if I'm wrong, but I seem to recall reading about them somewhere recently. I think it was thought that it would be a good idea of having a way to tell the driver in the event of a track circuit or signal failure that the route is indeed set up correctly, just not necessarily clear. Obviously takes out a lot of the hassle for the signaller having to manually key all the points, etc., and potential for error.
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PoSA 02/05/2015 at 08:36 #71624
maxand
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Quote:
Proceed on Sight Authority is a new concept which introduces an additional aspect to allow the signalman to authorise drivers to pass signals when they are at red due to influences within the interlocking. The signal will notionally be used where the route setting and locking function is still proved to be operable but a function such as train detection or lamp proving of a signal ahead may be failed. The authority will allow the driver to pass the signal and proceed at a speed slow enough that they may stop short of any obstruction (in common with other degraded modes of operation) (Wikipedia)
Added to glossary.

Last edited: 02/05/2015 at 09:50 by maxand
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PoSA 02/05/2015 at 12:31 #71631
Jay_G
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I believe the only area we have that has them at the minute is the thameslink core.

Most new schemes are getting them, the Streatham resignalling will have some POSA, London Bridge central side has it on the approach to London Bridge.

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PoSA 02/05/2015 at 12:33 #71632
JamesN
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East London Line has them aswell
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PoSA 02/05/2015 at 12:58 #71634
slatteryc
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Perhaps I am being obtuse; if the signalling has failed how can the PosA lights be trusted ? Does this not break a fail-safe principle ....
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PoSA 02/05/2015 at 13:27 #71635
JamesN
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" said:
Perhaps I am being obtuse; if the signalling has failed how can the PosA lights be trusted ? Does this not break a fail-safe principle ....
If all signalling has failed then the PoSA won't work, driver will get a red (or black) signal and current safeguards will stil hold.

PoSA aspect means exactly that: Proceed on Sight Authority - that is, the signalling equipment thinks there may be an obstruction on the line between here and the next signal. All the points in the route are set for you to get to the next signal, proceed cautiously. It's there to reduce signaller workload during failure scenarios. Now the signaller only needs to talk by and caution the first train through the failure, once that's happened had he has route locking he can clear the PoSA on approach of each subsequent train.

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PoSA 02/05/2015 at 13:54 #71636
Late Turn
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It also has the benefit of proving the route, removing a significant risk (of the route not being set correctly) from the process. Can the PoSA indication be used for the first train, after the driver has been given his instructions to examine the line?
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PoSA 02/05/2015 at 15:35 #71644
mfcooper
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" said:
How are PoSA signals activated on the panel / VDU?

On the WestCAD that used to be at Victoria (now at Three Bridges ROC) there was a separate keyboard button & on-screen button, which if either were pressed, the following route setting command would then set the POSA route.


" said:
Why aren't they more widely used?

They need to be installed before they can be used. The East London Line had them first when New Cross Gate signal box opened and the Thameslink Core had them added when Blackfriars was re-signalled.


" said:
Can the PoSA indication be used for the first train, after the driver has been given his instructions to examine the line?

As you already said,


" said:
It also has the benefit of proving the route, removing a significant risk (of the route not being set correctly) from the process.

So to keep the added safety of proving the route - yes.

And in the case of a Track Circuit Failure, after the first train has been spoken to to examine the line and then given the POSA aspect, subsequent trains only need the POSA aspect to proceed, rather than talking to the signaller each time. The signaller, however, has to make sure that the previous train has cleared the failure before giving the following train the POSA aspect.

Last edited: 02/05/2015 at 15:38 by mfcooper
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PoSA 02/05/2015 at 16:10 #71646
Muzer
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" said:
" said:
How are PoSA signals activated on the panel / VDU?

On the WestCAD that used to be at Victoria (now at Three Bridges ROC) there was a separate keyboard button & on-screen button, which if either were pressed, the following route setting command would then set the POSA route.

Ah, so if implemented in SimSig it would probably work similarly to the "reminder override" button?

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PoSA 02/05/2015 at 16:44 #71649
John
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" said:
Why aren't they more widely used?

" said:
They need to be installed before they can be used.

Thanks Matt

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PoSA 02/05/2015 at 16:55 #71650
GeoffM
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Also significantly increases interlocking logic size which impacts on how many signalling objects you can have per interlocking, which increases the number of interlockings, which increases the number of interlocking boundaries, which increases the amount of interlocking...
SimSig Boss
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PoSA 02/05/2015 at 17:08 #71651
Late Turn
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" said:
" said:
It also has the benefit of proving the route, removing a significant risk (of the route not being set correctly) from the process.

So to keep the added safety of proving the route - yes.

And in the case of a Track Circuit Failure, after the first train has been spoken to to examine the line and then given the POSA aspect, subsequent trains only need the POSA aspect to proceed, rather than talking to the signaller each time. The signaller, however, has to make sure that the previous train has cleared the failure before giving the following train the POSA aspect.

Thanks for confirming - I thought as much (it makes perfect sense), but I interpreted James' post as suggesting otherwise.

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PoSA 02/05/2015 at 17:19 #71652
John
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" said:
Also significantly increases interlocking logic size which impacts on how many signalling objects you can have per interlocking, which increases the number of interlockings, which increases the number of interlocking boundaries, which increases the amount of interlocking...
So I guess they are predominantly used in sensitive areas.

I noticed that with all the new signals being installed between Thornton Heath and Balham, only the signals at junctions have been equipped with PoSA, and was wondering if it was merely the cost factor that precluded their wider use.

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PoSA 02/05/2015 at 18:23 #71653
mfcooper
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" said:
I noticed that with all the new signals being installed between Thornton Heath and Balham, only the signals at junctions have been equipped with PoSA, and was wondering if it was merely the cost factor that precluded their wider use.
Only the signalling between Balham (exclusive) and Thornton Heath (exclusive) is to be transferred to Three Bridges ROC at some point (May 2016 or later). I assume that they are only replacing these signals. The last info I saw was that every signal would have POSA, but that was before I'd heard anything about Interlocking size (as per Geoff's post).

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PoSA 02/05/2015 at 18:53 #71654
Hawk777
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" said:
And in the case of a Track Circuit Failure, after the first train has been spoken to to examine the line and then given the POSA aspect, subsequent trains only need the POSA aspect to proceed, rather than talking to the signaller each time. The signaller, however, has to make sure that the previous train has cleared the failure before giving the following train the POSA aspect.
Sorry, but I don’t see why this is necessary. Isn’t the whole point of proceeding on sight that the train is able to stop short of any obstruction—such as the previous train that’s still in the section?

Last edited: 02/05/2015 at 18:54 by Hawk777
Reason: Improper quote markup

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PoSA 02/05/2015 at 19:05 #71655
mfcooper
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" said:
Sorry, but I don’t see why this is necessary. Isn’t the whole point of proceeding on sight that the train is able to stop short of any obstruction—such as the previous train that’s still in the section?
Two in a section is still frowned upon.

POSA's are only designed to quicken up the process of talking past a signal in certain failure circumstances by removing the need for voice communications (which is the most time-consuming part of the process). If a signaller were talking past a signal, they would make sure the previous train is clear before talking to the next train.

Last edited: 02/05/2015 at 19:06 by mfcooper
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PoSA 02/05/2015 at 19:37 #71656
Danny252
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" said:
" said:
Also significantly increases interlocking logic size which impacts on how many signalling objects you can have per interlocking, which increases the number of interlockings, which increases the number of interlocking boundaries, which increases the amount of interlocking...
So I guess they are predominantly used in sensitive areas.

I noticed that with all the new signals being installed between Thornton Heath and Balham, only the signals at junctions have been equipped with PoSA, and was wondering if it was merely the cost factor that precluded their wider use.
Well, if there's no junction, there's no points in the route to prove, so the added security would be fairly minimal - there's also generally less track circuits to fail on plain line.

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PoSA 02/05/2015 at 21:30 #71659
John
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" said:
" said:
" said:
Also significantly increases interlocking logic size which impacts on how many signalling objects you can have per interlocking, which increases the number of interlockings, which increases the number of interlocking boundaries, which increases the amount of interlocking...
So I guess they are predominantly used in sensitive areas.

I noticed that with all the new signals being installed between Thornton Heath and Balham, only the signals at junctions have been equipped with PoSA, and was wondering if it was merely the cost factor that precluded their wider use.
Well, if there's no junction, there's no points in the route to prove, so the added security would be fairly minimal - there's also generally less track circuits to fail on plain line.
True, but a track circuit failure on plain line at, for example, Streatham Common on the up fast still has the potential to cause a lot of problems, with fast line services being diverted to mingle with slow line traffic. PoSA would go some way to mitigating the effects.

But if, as Geoff says, it will exponentially increase the amount of interlockings and therefore cost, it's no surprise that they're not widely used.

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PoSA 02/05/2015 at 22:45 #71660
TimTamToe
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635 posts
" said:


True, but a track circuit failure on plain line at, for example, Streatham Common on the up fast still has the potential to cause a lot of problems, with fast line services being diverted to mingle with slow line traffic. PoSA would go some way to mitigating the effects.
Oh if only it could also help when there's conductor rail issues a bit further up the line too :whistle:

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PoSA 05/07/2015 at 18:11 #73928
collexions
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The imminent Victoria Phase II, Stage 1 scheme for Streatham workstation will have just seven PoSA signals.

The next TBROC workstation for Charing Cross (temp stage 'HL07'will have PoSA signals (initially disabled until full 2018 TL commissioning).

An imminent upgrade to Thameslink Core will implement new 'Emergency Point & Route Release' functionality, allowing the signaller/dispatcher to release 'failed' track circuits (inc. point TCs) in any applicable line of route. This will free subsequent sub-route locking, and allow for the core to be recovered more quickly in such circumstances, in-line with the use of said PoSA aspects.

The Thameslink Core, London Bridge Inner & Outer pod workstations TMS award has now been made, with initial work beginning in August.

Expect more PoSAs!

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PoSA 06/07/2015 at 11:26 #73946
Jersey_Mike
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" said:
" said:
Sorry, but I don’t see why this is necessary. Isn’t the whole point of proceeding on sight that the train is able to stop short of any obstruction—such as the previous train that’s still in the section?
Two in a section is still frowned upon.

POSA's are only designed to quicken up the process of talking past a signal in certain failure circumstances by removing the need for voice communications (which is the most time-consuming part of the process). If a signaller were talking past a signal, they would make sure the previous train is clear before talking to the next train.
Perhaps UK rail's chronic tardiness is due to a lack of operational flexibility? Entering occupied sections is one advantage railways should have over rapid transit operations.
Quote:

So I guess they are predominantly used in sensitive areas.

I noticed that with all the new signals being installed between Thornton Heath and Balham, only the signals at junctions have been equipped with PoSA, and was wondering if it was merely the cost factor that precluded their wider use.
In theory that is how it should be since signals between interlockings should be automatic.

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PoSA 06/07/2015 at 12:25 #73950
headshot119
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" said:

Quote:

So I guess they are predominantly used in sensitive areas.

I noticed that with all the new signals being installed between Thornton Heath and Balham, only the signals at junctions have been equipped with PoSA, and was wondering if it was merely the cost factor that precluded their wider use.
In theory that is how it should be since signals between interlockings should be automatic. ;)
Why should they be? Or are you referring to America practice? (Or perhaps an interlocking in America is different to that in the UK?)

I can think of plenty of UK interlockins where it simply isn't possible to have automatic signals between them, mainly due the point work involved. In some areas the size of an interlocking would have to be increased to a huge size in order to have automatic signals between it and neighboring interlockings.

"Passengers for New Lane, should be seated in the rear coach of the train " - Opinions are my own and not those of my employer
Last edited: 06/07/2015 at 12:28 by headshot119
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PoSA 06/07/2015 at 13:01 #73951
Steamer
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" said:
" said:
" said:
Sorry, but I don’t see why this is necessary. Isn’t the whole point of proceeding on sight that the train is able to stop short of any obstruction—such as the previous train that’s still in the section?
Two in a section is still frowned upon.

POSA's are only designed to quicken up the process of talking past a signal in certain failure circumstances by removing the need for voice communications (which is the most time-consuming part of the process). If a signaller were talking past a signal, they would make sure the previous train is clear before talking to the next train.
Perhaps UK rail's chronic tardiness is due to a lack of operational flexibility? Entering occupied sections is one advantage railways should have over rapid transit operations.
Wrong way round- light rail generally uses the "Line Of Sight" principle, thanks to their lower speeds and better braking.


Quote:
(Or perhaps an interlocking in America is different to that in the UK?)
From previous threads, I've surmised that they are, and that different rules apply within and outside them. Drivers are also aware of their boundaries.

"Don't stress/ relax/ let life roll off your backs./ Except for death and paying taxes/ everything in life.../ is only for now." (Avenue Q)
Last edited: 06/07/2015 at 13:02 by Steamer
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PoSA 06/07/2015 at 16:01 #73957
Jersey_Mike
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" said:

Why should they be? Or are you referring to America practice? (Or perhaps an interlocking in America is different to that in the UK?)

I can think of plenty of UK interlockins where it simply isn't possible to have automatic signals between them, mainly due the point work involved. In some areas the size of an interlocking would have to be increased to a huge size in order to have automatic signals between it and neighboring interlockings.
When someone said "junction" I assumed that meant point work with the implication that there are a number of interlocked signals between junctions without associated points or what we refer to as "interlocking appliances". Eliminating unnecessary interlocking is a good way to reduce costs.

" said:
Wrong way round- light rail generally uses the "Line Of Sight" principle, thanks to their lower speeds and better braking.
You'd think that, but all new (and some older) light rail systems are being outfitted with ATC outside of street operation. It reduces the requisite skill of the work force and proactively eliminates work to rule job actions as seen with the Boston light rail system.

" said:
" said:

(Or perhaps an interlocking in America is different to that in the UK?)
From previous threads, I've surmised that they are, and that different rules apply within and outside them. Drivers are also aware of their boundaries.
Here is a fun chart from the former Tower 55 crossing in Fort Worth that illustrates the concept of interlocking limits. Tower 55 used to be all one big interlocking, but it is simpler to split it into many logical interlockings, probably for testing and modification purposes.

Regarding Call-on signals (or PoSA signals as someone saw fit to rename them), it was typical for many North American railroads to omit them when they initially signaled lines with CTC due to the extra expense of interlocking them with traffic control (a call-on is permissible with flow of traffic, but is not if flow of traffic is set against the requested movement) In that case only wealthier railroads would install this capability. Other railroads would install it only for movements entering a potentially occupied siding, but not for movements leaving the siding (or main track) into a single track section.

Freight railroad CSX has been busy reversing its previous policy of not installing call-on capability in all of its resignaling projects. This has lead to the awkward situation of needing 4-lamp signal heads as CSX predecessor Seaboard System saw call-on as a luxury and provided for it with special lunar white signal aspects that were only used around yards or in other special circumstances. Out west the same was frequently true, but by adopting *R* aspects for call-on they can make do with 3-lamp heads.

Last edited: 06/07/2015 at 16:01 by Jersey_Mike
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