Q1.
Yes you could put the PLJI on signal 11 and not have 15 at all.
Depending on distance to 21, probably 11 would be a 2 aspect Red/ Green and a Yellow/Green signal would be needed placed braking distance prior to 21.
Wouldn't want the protecting signal too far from the points it protects though, so alternatively 11 could be positioned in position of 15.
Probably driver from station A could see it and choose not to leave the station whilst at red; however it is generally not necessary that there be a signal at the end of every platform. Where a station platform is mid-section, then the driver just restarts their train continuing in the same direction as before. No signs, no instuctions (other than where there has to be a train guard to confirm all doors closed safely if they are not power operated, interlocked with the train and the driver has visibillity of external of train via mirrors or CCTV monitors).
There is a need though to provide a signal "at place of reversal"- the driver cannot restart in the opposite direction to the origial one without a signal which authorises this. So in this case since trains terminate and reverse direction at station A ,then it would be a good idea to have signals at the platform end or at least relatively close to it, certainly visible from it.
Q2.
"Approach Release" means that the signal is deliberately initially held at a more restrictive aspect than its "true" aspect relative to the aspect displayed by the one beyond it. It is a disadvantage of "route signalling", it is over-restrictive and reduces capacity and indeed can be a "SPAD trap" for the driver. However it is necessary because the alternative of not doing it is worse!
Consider the situation if this were not applied- a signal such as 21 would show Green with PLJI when routed up to 23 at Green. That would itself mean that the signal 17 would also show Green. The driver of the train would then get the same aspect sequence initially as if 21 had been set on the straight route via 25. Therefore driver would be coming fast and see 21 in the distance at Green still expecting to go straight on; then much later the driver would notice that it also displayed a PLJI and that train needed to go via the loop points. These may only be 25mph rather than say 80mph for the straight route; driver would not be able to slow sufficiently in the available length and hence the train would overspeed at the junction and would likely derail.
Instead we hold 21 at Red, so that driver gets a Yellow at 17 and therefore expects to stop at 21. Therefore when we eventually allow 21 to clear (i.e. "approach release from red") the driver may be typically 400m from it with the brakes on expecting to stop at it, but then sees it clear to Green + PLJI and thus accelerates again so that pass over the points at the safe speed. Over restrictive but safe. Not too bad for 25mph points but much less desirale if the points permit 50mph divergence- the signalling would prevent this opportunity being achieved in practice. Hence in such situations would probably implement "approach release from yellow"; however even this is not ideal but it is basically what the UK are stuck with until we change to ETCS which is a from of "speed signalling".
Does this help you understand?
Yes you could put the PLJI on signal 11 and not have 15 at all.
Depending on distance to 21, probably 11 would be a 2 aspect Red/ Green and a Yellow/Green signal would be needed placed braking distance prior to 21.
Wouldn't want the protecting signal too far from the points it protects though, so alternatively 11 could be positioned in position of 15.
Probably driver from station A could see it and choose not to leave the station whilst at red; however it is generally not necessary that there be a signal at the end of every platform. Where a station platform is mid-section, then the driver just restarts their train continuing in the same direction as before. No signs, no instuctions (other than where there has to be a train guard to confirm all doors closed safely if they are not power operated, interlocked with the train and the driver has visibillity of external of train via mirrors or CCTV monitors).
There is a need though to provide a signal "at place of reversal"- the driver cannot restart in the opposite direction to the origial one without a signal which authorises this. So in this case since trains terminate and reverse direction at station A ,then it would be a good idea to have signals at the platform end or at least relatively close to it, certainly visible from it.
Q2.
"Approach Release" means that the signal is deliberately initially held at a more restrictive aspect than its "true" aspect relative to the aspect displayed by the one beyond it. It is a disadvantage of "route signalling", it is over-restrictive and reduces capacity and indeed can be a "SPAD trap" for the driver. However it is necessary because the alternative of not doing it is worse!
Consider the situation if this were not applied- a signal such as 21 would show Green with PLJI when routed up to 23 at Green. That would itself mean that the signal 17 would also show Green. The driver of the train would then get the same aspect sequence initially as if 21 had been set on the straight route via 25. Therefore driver would be coming fast and see 21 in the distance at Green still expecting to go straight on; then much later the driver would notice that it also displayed a PLJI and that train needed to go via the loop points. These may only be 25mph rather than say 80mph for the straight route; driver would not be able to slow sufficiently in the available length and hence the train would overspeed at the junction and would likely derail.
Instead we hold 21 at Red, so that driver gets a Yellow at 17 and therefore expects to stop at 21. Therefore when we eventually allow 21 to clear (i.e. "approach release from red") the driver may be typically 400m from it with the brakes on expecting to stop at it, but then sees it clear to Green + PLJI and thus accelerates again so that pass over the points at the safe speed. Over restrictive but safe. Not too bad for 25mph points but much less desirale if the points permit 50mph divergence- the signalling would prevent this opportunity being achieved in practice. Hence in such situations would probably implement "approach release from yellow"; however even this is not ideal but it is basically what the UK are stuck with until we change to ETCS which is a from of "speed signalling".
Does this help you understand?
(15-11-2011, 08:03 AM)onestrangeday Wrote: Hi PJW:
Sure no worries, I am not in hurry, take your time. I just only ask for making sure my thinking is on the track or not.
Once again thanks for your reply.
thanks
I have few questions though:
"a) 11 is shown as a Red/Green not far in rear of 15 (similarly 13 and 17). Probably best not to provide 11- there is no absolute need to have a platform starting signal. If really need 11, then would need to make it "preceeded" bt 15; i.e. prevent 11 for clearing unless 15 already cleared."
Q1: If in this case the platform is very close to the points area, can I eliminate the junction indicator, instead place at the platform with the starting signal ?
or if you eliminate the platform starting signal, how do you show to the driver when to proceed ? (how does this done in UK ?) with signs, or order from train controller ?
Q2:
"I have shown on the attachment how to show the "MAR" Approach Release from Red for those routes which are the slower speed divergence through points."
what is "MAR" Approach release ?
thanks
PJW