PoSA

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)

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 ....

How are PoSA signals activated on the panel / VDU?

Why aren't they more widely used?

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

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

" 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.

Why aren't they more widely used?

They need to be installed before they can be used.

" 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.

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...

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.

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?

" 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.

" 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.

" 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.

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.

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. ;)

" 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.

(Or perhaps an interlocking in America is different to that in the UK?)

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.

Wrong way round- light rail generally uses the "Line Of Sight" principle, thanks to their lower speeds and better braking.

" 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.