poSA Indicator

[Varalakshmi P]

Hi PJW,

Thanks for remembering & Thank you very much for the reply.

Thanks & regards
Varalakshmi Penneru

Hi PJW

Can you please help me to get the difference between poSA & Call-on aspect? On which degraded situations we can go for poSA?

Thanks & regards
Varalakshmi Penneru

Hi Varalakshmi- a long time since I have heard from you!

Call-on is a "normal aspect"; it is for passenger trains to be signalled into an already occupied platform. This is necessary when trains need to join. The driver sees the main signal at red but with the illumination of a PL (Position Light= two diagonally displayed white lights).

Joining train is a timetabled requirement in some places; for example at Redhill in the morning a train of 8 cars comes in from Tonbridge, then soon afterwards a train of 4 cars arrives from Reigate. This stops about 10m prior and then draws forward slowly and then the units couple up; the whole train then continues to London, needing only one driver and only one train path in the rush hour.
The call-on is the aspect used to allow the second train to proceed cautiously into an occupied platform line; the yellow aspect cannot be given as the tracks are not clear.
Call-ons are provided at many busy stations; they can also be used to get a second train into the same platform even if the trains are not to join up but merely to cope with more trains needing to be there at the same time than there are platforms available. This usage is non-preferred nowadays although may be perpetuated where has history of use.

The POSA is an aspect (which looks similar except that the two diagonal whilte lights flash in unison rather than being steadily illuminated) provided explicitly to mitigate the effects of a failure that prevents the clearance of the normal signal. It is a better alternative to the signaller verbally authorising a signal to be passed at danger. The aspect guarantees a "wheeled path" (i.e. all the points in route to exit signal detected in position) but very little else. It would be used if the usual signal is unable to clear bexcause of some failure, perhaps detection of points within the overlap or on the flank of the route, perhaps the TPWS of the exit signal, or a track circuit failure somewhere.

In terms of the Rulebook the driver in each case knows that they must proceed at a speed at which they can stop short of any obstruction, certainly not exceeding 15mph. In that respect there is no great difference. However the driver would only expect to receive a call-on if such working is a feature of the area; indeed where there is cab radio I would expect the signaller would contact the driver before giving a call-on aspect if that was not what would normally be given for that train in the timetable. The POSA would be unusual but the driver would understand that there must be some failure in the area requiring the resort to degraded mode working. Actually there are very few POSA yet commissioned on NR, but such degraded mode aspects are more common in Europe. The reason for putting the bare minimum of infrastructure proving within the coontrols of a PoSA is to ensure that it can be used in as many as possible failure scenarios.

The call-on is given when there is an occupied track circuit as the result of the first train; the POSA is given when the signaller expects that the tracks should be clear but can't guarantee this state. The driver receiving a call-on will definitely expect to encounter a train probably right at the end of the route; the driver receiving a POSA won't really anticipate encountering onbe but knows that this is a possibility of this anywhere along the route.