• Our booking engine at tickets.railforums.co.uk (powered by TrainSplit) helps support the running of the forum with every ticket purchase! Find out more and ask any questions/give us feedback in this thread!

PoSA signals - eh?

Status
Not open for further replies.

martin2345uk

Established Member
Joined
21 Sep 2011
Messages
2,052
Location
Essex
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?
 
Last edited:
Sponsor Post - registered members do not see these adverts; click here to register, or click here to log in
R

RailUK Forums

43066

Established Member
Joined
24 Nov 2019
Messages
9,217
Location
London
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

Veteran Member
Joined
7 Apr 2010
Messages
31,263
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

Established Member
Joined
10 Jun 2005
Messages
5,826
"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

On Moderation
Joined
21 Oct 2009
Messages
2,810
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

Established Member
Joined
24 Nov 2019
Messages
9,217
Location
London
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

Veteran Member
Joined
7 Apr 2010
Messages
31,263
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

Established Member
Joined
21 Sep 2011
Messages
2,052
Location
Essex
Thanks for the replies, I doubt I will encounter them any time soon but it's good to know...
 

Bald Rick

Veteran Member
Joined
28 Sep 2010
Messages
29,070
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

Member
Joined
27 Oct 2015
Messages
1,176
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

Member
Joined
26 Apr 2010
Messages
358
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

Veteran Member
Joined
28 Sep 2010
Messages
29,070
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

Member
Joined
17 Aug 2018
Messages
860
So what's the point in them if they act the same as position lights? What's the difference?
 

Watershed

Veteran Member
Associate Staff
Senior Fares Advisor
Joined
26 Sep 2020
Messages
11,945
Location
UK
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

Member
Joined
17 Aug 2018
Messages
860
I thought that's what position lights were all about anyway tho? Or is that just a FP thing, not a PP thing?
 

Ianno87

Veteran Member
Joined
3 May 2015
Messages
15,215
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

Member
Joined
17 Aug 2018
Messages
860
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

Veteran Member
Joined
7 Apr 2010
Messages
31,263
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

Veteran Member
Joined
7 Apr 2010
Messages
31,263
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

Established Member
Joined
24 Nov 2019
Messages
9,217
Location
London
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

Established Member
Joined
4 Dec 2006
Messages
4,223
Location
DTOS A or B
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

Member
Joined
13 Jun 2016
Messages
170
Location
Norwich
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.
 
Status
Not open for further replies.

Top