PoSA signals - eh?

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martin2345uk

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How common are Proceed on Sight Authority signals?

I've never seen one, and the whole module was completely skipped during our rules course, so I know very little about them - from what I understand from the module, they're basically position-lights that flash meaning "even though this signal has a fault, you can pass it at caution, be prepared to stop etc etc"..? Is there any more to them?

Where one is installed, can it also function as a standard position light i.e. not flash?
 
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43066

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How common as Proceed on Sight Authority signals?

I've never seen one, and the whole module was completely skipped during our rules course, so I know very little about them - from what I understand from the module, they're basically position-lights that flash meaning "even though this signal has a fault, you can pass it at caution, be prepared to stop etc etc"..? Is there any more to them?

Where one is installed, can it also function as a standard position light i.e. not flash?

Treat it like a dummy. If it flashes, proceed at caution. They can also serve as normal position light signals, so ditto if it shows two steady whites.

Not common, unless you drive through the Thameslink core.
 

swt_passenger

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The East London Line predates Thameslink I thought?

Searching the forum for mentions of “POSA” suggests there are a few other installations here and there, although Thameslink seemed to get most of the initial coverage for some reason. Was there more internal PR for the Thameslink core changes?
 

Tomnick

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"even though this signal has a fault..."
It's more "a fault with something else is preventing this signal from being cleared", e.g. a track circuit failure in the route to the next signal, lamp out at the next signal, that sort of thing. Sometimes it's not necessary for the signalman to speak to the driver first, but even if it is necessary, having the POSA lit means that the movement's still benefitting from the safety of the interlocking.
 

Surreytraveller

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Basically saves the time taken for the driver and signaller to converse - basically means pass this signal at danger, proceed with caution etc etc. Points will be proved.
Some also on the approaches to London Bridge, and there's one in the Three Bridges area somewhere
 

43066

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The East London Line predates Thameslink I thought?

Searching the forum for mentions of “POSA” suggests there are a few other installations here and there, although Thameslink seemed to get most of the initial coverage for some reason. Was there more internal PR for the Thameslink core changes?

I must admit I was unaware they were installed on the ELL. TLK seemed to trumpet them as a way to minimise delays, but in reality they were used exceptionally rarely in my time driving through the core. Also extend as far back as London Bridge.

Perhaps I would could have more accurate to say that they’re rarely encountered, even if you drive over a route equipped with them, because they’re only used during degraded workings.
 

swt_passenger

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I must admit I was unaware they were installed on the ELL. TLK seemed to trumpet them as a way to minimise delays, but in reality they were used exceptionally rarely in my time driving through the core. Also extend as far back as London Bridge.

Perhaps I would could have more accurate to say that they’re rarely encountered, even if you drive over a route equipped with them, because they’re only used during degraded workings.
Yes, I saw a post somewhere wondering if the ELL had ever used them...
 

martin2345uk

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Thanks for the replies, I doubt I will encounter them any time soon but it's good to know...
 

Bald Rick

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The East London Line predates Thameslink I thought?

Searching the forum for mentions of “POSA” suggests there are a few other installations here and there, although Thameslink seemed to get most of the initial coverage for some reason. Was there more internal PR for the Thameslink core changes?

ELL was first, but Thameslink was the first on NR infrastructure.

Basically saves the time taken for the driver and signaller to converse - basically means pass this signal at danger, proceed with caution etc etc. Points will be proved.
Some also on the approaches to London Bridge, and there's one in the Three Bridges area somewhere

Gatwick, IIRC, and there’s possibly a few in the ex London Bridge control area on the SE side.


I must admit I was unaware they were installed on the ELL. TLK seemed to trumpet them as a way to minimise delays, but in reality they were used exceptionally rarely in my time driving through the core. Also extend as far back as London Bridge.

They paid for themselves in the first incident that required their use. I reckon they are used once a month on average, but it happens in clusters - a few times in a couple of weeks then nothing for months. About half the use has been down to the seemingly intractable flooding in Clerkenwell.
 

Sunset route

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There are no POSA signals under the control of TBASC at Gatwick or anywhere else, they are proposed for Gatwick as an add to the re-locking scheme when platform 7 was introduced but nothing came of it.
 

Skoodle

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Yes the East London Line has had them since day 1, on every signal protecting a junction or set of points. I've only seen them in use once, but have been used a handful of times.
 

Bald Rick

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There are no POSA signals under the control of TBASC at Gatwick or anywhere else, they are proposed for Gatwick as an add to the re-locking scheme when platform 7 was introduced but nothing came of it.

Ah - that’s where I saw it proposed. Apologies.
 

4F89

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So what's the point in them if they act the same as position lights? What's the difference?
 

Watershed

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So what's the point in them if they act the same as position lights? What's the difference?
They can be used in passenger service without special authority being needed.
 

4F89

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I thought that's what position lights were all about anyway tho? Or is that just a FP thing, not a PP thing?
 

Ianno87

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They can be used in passenger service without special authority being needed.

I thought that's what position lights were all about anyway tho? Or is that just a FP thing, not a PP thing?

POSAs effectively replace having to be "talked past" signals in the event of a failure. They are effectively a signalling "failure mode" whereas position lights are "normal operation".

In terms of what they mean, they mean a very similar thing - "proceed into section and be prepared to stop short of an obstruction", however for a POSA this could be anything anywhere, but a position light the "obstruction" will be known (e.g. another train part occupying the platform).
 

4F89

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POSAs effectively replace having to be "talked past" signals in the event of a failure. They are effectively a signalling "failure mode" whereas position lights are "normal operation".

In terms of what they mean, they mean a very similar thing - "proceed into section and be prepared to stop short of an obstruction", however for a POSA this could be anything anywhere, but a position light the "obstruction" will be known (e.g. another train part occupying the platform).
Fair enough.
 

swt_passenger

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POSAs effectively replace having to be "talked past" signals in the event of a failure. They are effectively a signalling "failure mode" whereas position lights are "normal operation".

In terms of what they mean, they mean a very similar thing - "proceed into section and be prepared to stop short of an obstruction", however for a POSA this could be anything anywhere, but a position light the "obstruction" will be known (e.g. another train part occupying the platform).
Isn’t a key difference that you could have a number of POSA indications in sequence?
 

swt_passenger

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BTW I looked in the public version of the Kent/Sussex/Wessex sectional appendix. A search using “POSA” comes up with various routes/areas that are fitted, seems to be bounded by Farringdon, Charing Cross, Cannon St, Herne Hill, Windmill Bridge, and “Lucas St Tunnels” (near New Cross?). So it seems to cover quite an area, presumably only the routes that impact on Thameslink operations though?

I haven’t checked the LNE equivalent, presumably it will list the northern limits though.
 

43066

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and “Lucas St Tunnels” (near New Cross?). So it seems to cover quite an area, presumably only the routes that impact on Thameslink operations though?

Yes - between New Cross and St. Johns. Makes sense as TL services can be diverted that way if the Greenwich line is closed.
 

westcoaster

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There are no POSA signals under the control of TBASC at Gatwick or anywhere else, they are proposed for Gatwick as an add to the re-locking scheme when platform 7 was introduced but nothing came of it.
Not quite true Three Bridges ROC Streatham workstation has them atStreatham north junction.

Only the first train through the affected section needs to be told of there use by the signaller. All further trains proceed as fas the line can be seen clear. In a tunnel and going around a very sharp bend means you go slow very slow.

In the core and elsewhere the thing to remember is that TPWS train stop is suppressed but TPWS overspeed is still active.
 

Shwam3

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Not quite true Three Bridges ROC Streatham workstation has them atStreatham north junction.

Only the first train through the affected section needs to be told of there use by the signaller. All further trains proceed as fas the line can be seen clear. In a tunnel and going around a very sharp bend means you go slow very slow.

In the core and elsewhere the thing to remember is that TPWS train stop is suppressed but TPWS overspeed is still active.
Three Bridges ASC is not the same as Three Bridges ROC.
 
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