PoSA or APLS??

[BrightonBeach]

This may seem a silly question…

However, what is the point of a PoSA where an associated position light signal (that also looks identical to a PoSA when not lit) when lit, also allows you to pass the red aspect associated with it??

It seems a flashing PoSA, and a solid white lit APLS have the same meaning?

Be kind I’m only a few weeks in…

Moderator note:
POSA - Proceed On Sight Authority
APLS - Associated Position Light Signal

 

[irish_driver]

Very quickly, the APLS provides authority to 'pass' a signal to the point of obstruction where that function is specifically designed - permissive platforms, entry to Yards and Sidings.etc.

A PoSA is slightly different. It's provided at a signal where it's only function is to act in the result of a failure of the signal/associated route indicator or track circuit(s) - it cannot be used for permissive working, although your instructions as a Driver are the same, proceed past the signal, being able to stop short of an obstruction. A PoSA is simply there to speed up the process of passing a defective signal.

 

[davidhg]

Any chance of some explanation please? Always helpful in the initial post to use full words rather than acronyms.

Particularly with technical terms that a non-professional would not know.

 

[FR510]

POSA - Proceed On Sight Authority

APLS - Associated Position Light Signal

 

[Wilts Wanderer]

I’m aware of PoSAs being used in the Thameslink tunnels where it would understandably take ages to solve signal failures etc and traffic density means congestion builds rapidly. Where else are they used?

 

[zwk500]

I’m aware of PoSAs being used in the Thameslink tunnels where it would understandably take ages to solve signal failures etc and traffic density means congestion builds rapidly. Where else are they used?

AFAIK, nowhere else.

 

[Shwam3]

Where else are they used?

In the Thameslink tunnels as mentioned, plus the lines from Blackfriars to New Cross & New Cross Gate and also on the East London Line between Highbury & Islington and New Cross/New Cross Gate.

 

[Surreytraveller]

AFAIK, nowhere else.

Gatwick.

 

[Wilts Wanderer]

Thanks both. So an extremely specialised usage at the moment.

 

[godfreycomplex]

Thanks both. So an extremely specialised usage at the moment.

And likely to remain so by the look of it

 

[Supercoss]

PoSA working March 2018

London St Pancras Low Level - Proceed On Site Authority signalling https://www.rssb.co.uk/rgs/rulebooks/GERT8000-PoSA%20Iss%203.pdf

vimeo.com

Platform A at St Pancras low level example of POSA working

 

[zwk500]

Gatwick.

Really? Neither Gatwick or the ELL are mentioned as PoSA areas in the Sectional Appendix.

(correcting my earlier AFIAK post: ) Sectional Appendix only lists PoSA in the Table A from Kentish Town to just north of Elephant and Castle, Blackfriars to Metropolitan Junction, Waterloo East to New Cross, London Bridge to New Cross Gate, and the Canal Tunnels.

 

[Harpo]

I’d love to hear a Japanese view on the philosophy of installing additional infrastructure rather than making sure what you’ve got actually works.

 

[rheingold103]

On HS1 (CTRL) a single lens signal head having a function equivalent to the POSA is provided at Non-passable Block Markers - following the French standard practice of the Œilleton.

 

[martin2345uk]

On HS1 (CTRL) a single lens signal head having a function equivalent to the POSA is provided at Non-passable Block Markers - following the French standard practice of the Œilleton.

HS1 uses the uk style two-lens position lights in place of the “oeilleton” on the French side.
Shame they couldn’t have standardised with the French and ET sides when they built it, maybe they thought it would be too confusing for UK drivers!

 

[godfreycomplex]

HS1 uses the uk style two-lens position lights in place of the “oeilleton” on the French side.
Shame they couldn’t have standardised with the French and ET sides when they built it, maybe they thought it would be too confusing for UK drivers!

Getting spares was the main reason for that I believe

 

[Sunset route]

Gatwick.

Gatwick does not have POSA signals, although they were looking at installing them not long after platform 7 was commissioned. As a signaller I represented my colleagues views on the scheme, but over time the meetings dried up and the scheme has sunk without a trace since.

 

[davyp]

POSA - Proceed On Sight Authority

APLS - Associated Position Light Signal

Thank you

 

[Belperpete]

I’d love to hear a Japanese view on the philosophy of installing additional infrastructure rather than making sure what you’ve got actually works.

I am not sure that they use any additional infrastructure. The sub aspects are simply connected to a flashable output rather than a non-flashable output. They do require significant extra interlocking data though. I was under the impression that the data had been designed and tested for Gatwick, so I am surprised if it wasn't actually commissioned. It would however explain why we found it very difficult to get any feedback when they were being considered for other schemes.

== Doublepost prevention - post automatically merged: 1 Sep 2025 ==

This may seem a silly question…

However, what is the point of a PoSA where an associated position light signal (that also looks identical to a PoSA when not lit) when lit, also allows you to pass the red aspect associated with it??

As I recall, PoSAs were invented to save the signaller having to verbally authorise a driver to pass a signal at red. This can take appreciable time, and on a high capacity line like the Thameslink core, the cumulative delays of each successive train having to be verbally cautioned would soon add up.

A driver of a passenger train unexpectedly getting a subsidiary "call-on" would likely stop to query it with the signaller, which would negate the time saving, which is why a different aspect was felt necessary.

So whilst the actions taken by the driver may be similar, the meaning is subtly different. I would hope that a driver taking a POSA would be more alert to things like points not fully over, disconnected rail bonds and the like, than might be the case when taking a standard shunt route.

 

[Tester]

So whilst the actions taken by the driver may be similar, the meaning is subtly different. I would hope that a driver taking a POSA would be more alert to things like points not fully over, disconnected rail bonds and the like, than might be the case when taking a standard shunt route.

Rest assured that no POSA will be given if the points are not fully over!

 

[Paul AC]

Presumably the POSA is roughly equivalent to the permissive red you get with French signalling - steady red at a Franchissable (permissive) signal (plated F), or a steady single red with white marker light at a Non-franchissable (non-permissive) signal (plated Nf), both of which (I recall) tell the driver to stop at the signal then proceed 'on sight'. Also as I recall a flashing red at a Franchissable signal permits the driver to proceed on sight without stopping first.

 

[43066]

So whilst the actions taken by the driver may be similar, the meaning is subtly different. I would hope that a driver taking a POSA would be more alert to things like points not fully over, disconnected rail bonds and the like, than might be the case when taking a standard shunt route.

Rest assured that no POSA will be given if the points are not fully over!

Indeed. I’d expect no proceed aspect to be given at all if there was any risk of the route not being correctly secured. POSAs are as discussed above to enable trains to continue operating in degraded conditions in high traffic areas, without endless calls to the signaller. The meaning is essentially the same as a steady white position light - ie proceed on sight and stop short of any obstruction.

HS1 uses the uk style two-lens position lights in place of the “oeilleton” on the French side.
Shame they couldn’t have standardised with the French and ET sides when they built it, maybe they thought it would be too confusing for UK drivers!

Also somewhat odd that the position lights on the marker boards flash while the ones on the colour light signals outside St P are only capable of solid whites.

Also interesting that the forms are all based on French, yet is wasn’t found acceptable to standardise the lettering for the passable and non passable marker boards - I gather that was to avoid confusing the Brits .

 

[FR510]

A POSA signal flashing tells the driver they can pass the associated signal at danger without needing to contact the signaller.

The driver will know that any points are set correctly for their train. They will also assume the next signal will be at danger (although it may not be) and be ready to stop at the next signal.

 

[a340egkk]

PoSA also present on much of the signalling around London Bridge not just the TL core and lines

 

[Nicholas Lewis]

PoSA also present on much of the signalling around London Bridge not just the TL core and lines

Are SE and Southern trained to use them or is it just Tlk?

 

[westcoaster]

Are SE and Southern trained to use them or is it just Tlk?

Any driver that signs the route will know about them,

== Doublepost prevention - post automatically merged: 2 Sep 2025 ==

PoSA also present on much of the signalling around London Bridge not just the TL core and lines

Also present around both Steatham junctions, and Sutton iirc.

If a POSA signal is active and the signal has a route indicator or theatre box these will still illuminate.

POSA working is also compatibe with ETCS and the driver will be offered on site mode.

Also of note is that whilst running under POSA signals TPWS overspeed loops are still armed.