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Hello Peter,
Please go through my answer for the aspect sequence question for 2004.
The one issue in this question is the transition for two 3 aspect signals to 4 aspect which in my view do not need approach release since trains are starting from a stand. Hence I have put down my rationale for doing so on the chart. Please let me know if this is correct as this matter is not mentioned/addressed in Appendix S which you posted on this website.
Thank you again for your time and attention
Regards
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Since 109 and 121 cannot be approached, you should not have shown the overlap associated with their Reds!
As explained earlier, the issue is not re approach reease but the adequacy of the braking from the following signal at single Yellow to the Red beyond. Considereing trains from 121 which has to observe the 30kph over 210 points- it has some 550m over which it technically could be accelerating on the approach to 147 (but if we assume that the train itself would be at least 50m then that reduces to 500m). From 147 to 149 is 650m and what we really need to know is whether that distance is at least braking distance from whatever speed can be attained by acceleration upon clearing 210 points. The bad news is that the brake rate assumed by this plan is certainly not to regular 0.5m per second per second- indeed the rate at 30kph is less than half that at 10kph. This is probably more realistic than the normal module 2 assumption but makes it difficult for us to know what to do here. Certainly the easy assumption that the acceleration rate will be less than the brake rate is not enough to make it evident that all will be ok.
You could leave the aspect sequence drawn as you have and just blandly state the assumption that 147 - 149 represents sufficient braking at attainable speed from 121 since otherwise plan should have shown 121 as 4 aspect.
Alternatively you would have to decide to impose approach release on 147 [NOTE 147 not 121!]:
"(BS or BR or BM)occ or (210R or 209N.210N) or 149 OFF"
Hence when train coming from 127/125/123 the point conditions mean that it is not approach released yet when coming from 121 it is, but only when 149 is at Red. This would then ensure that driver gets a yellow on 121, then sees 147 at Yellow before the Red at 149 yet if 147 could show at least Double Yellow the 121 would show Green. This looks BAD SIGNALLING to me, when the real answer in that situation would be to make 121 a 4 aspect signal; but we can only do the aspect sequence chart for the layout we are given!
Don't have so much of an issue with 109; whatever the aspect on 123/125/127 the driver knows that they must be regulating their speed to pass over 208-210 at 30kph and so they really can't be trapped into travelling too fast to be able to stop at 147. (Do note that trains could well be moving, albeit at 20kph when approaching 109 from 103- so not quite true that staring fro a stand).
Apart from that the one definite error on your chart was the delayed yellow on 111 up to the ROL beyond 123 has been given the same approach release as for the MAR routes. BE track looks like 400m long and so 10 seconds seems fine for PLJI release; however it is far too short for a Warning class route. The ROL is 125m long so the approaching train need not be "nearly to a stnand" at 111, but it must be close to it travelling say at 30kph so I suggest about 30 seconds- certainly significantly different to the usual approach release.
The other thing is that it is vrtually impossible to see "what is what" in the centre of your chart. It is as bad as attempting to folow the tracks on a PCB design- it certainly does not show clearly the aspect sequence as an overall picture. Personally I do not like vertical lines as can't tell at a glance which way the thing reads as converging / diverging- it is not too bad when simple and well separated as it is beyond 121 etc., but in this area the convergence of 109 and 11 gets rather mixed up with the divergence beyond 111- it isn't immediately evident whether 109 does read to 127 for example. I know it is difficult in exam conditions but you'd loose some marks- its a bit of evidence to the examiners is that you hadn't planned before getting stuck-in; ideally for example the horizontal lines from 111 would go to 123 rather than 125 but that would mean that more space should have been left at the top and less at the bottom of the sheet.
One ip that can help in such areas is you could have converged he "similar sequence" from 123/125/127 into one set of 3 lines, just keeping the pair of lines from 121 to converge closer to the destination signal 147. As well as being clearer it would have saved some width and thus given more horizontal room in the centre part of the diagram.
Overall though not bad; I've seen far worse. If you'd got the more restrictive release time for the Warning route I reckon that it might have scored a credit
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04-08-2008, 08:57 PM
(This post was last modified: 11-08-2008, 06:59 PM by PJW.)
Since 109 and 121 cannot be approached, you should not have shown the overlap associated with their Reds!
As explained earlier, the issue is not re approach release but the adequacy of the braking from the following signal at single Yellow to the Red beyond. Considering a train from 121 (which has to observe the 30kph over 210 points), it has some 550m over which it technically could be accelerating on the approach to 147 (but if we assume that the train itself would be at least 50m then that reduces to 500m). From 147 to 149 is 650m and what we really need to know is whether that distance is at least braking distance from whatever speed can be attained by acceleration upon clearing 210 points. The bad news is that the brake rate assumed by this plan is certainly not the constant 0.5m per second per second that the IRSE exam so often assumes- indeed the rate at 30kph is less than half the rate at 10kph, so certainly not linear. This is probably more realistic than the normal module 2 assumption but makes it difficult for us to know what to do here. Certainly the easy assumption that the acceleration rate will be less than the brake rate is not enough to make it evident that all will be ok.
You could leave the aspect sequence drawn as you have and just blandly state the assumption that 147 - 149 represents sufficient braking at attainable speed from 121 since otherwise plan should have shown 121 as 4 aspect.
Alternatively you would have to decide to impose approach release on 147 [NOTE 147 not 121!]:
"(BS or BR or BM)occ or (210R or 209N.210N) or 149 OFF"
Hence when train coming from 127/125/123 the point conditions mean that it is not approach released yet when coming from 121 it is, but only when 149 is at Red. This would then ensure that driver gets a yellow on 121, then would see 147 at Yellow before the Red at 149, yet if 147 could show at least Double Yellow then 121 would show Green. This looks BAD SIGNALLING to me, when the real answer in that situation would be to make 121 a 4 aspect signal; but we can only do the aspect sequence chart for the layout we are given!
Don't have so much of an issue with 109; whatever the aspect on 123/125/127 the driver knows that they must be regulating their speed to pass over 208-210 at 30kph and so they really can't be trapped into travelling too fast to be able to stop at 147. (Do note that trains could well be moving, albeit at 20kph, when approaching 109 from 103- so not quite true that staring from a stand).
Apart from that, the one definite error on your chart was the delayed yellow on 111 up to the ROL beyond 123 has been given the same approach release as for the MAR routes. BE track looks like 400m long and so 10 seconds seems fine for PLJI release; however it is far too short for a Warning class route. The ROL is 125m long so the approaching train need not be "nearly to a stand" at 111, but it must be close to it travelling say at 30kph so I suggest about 30 seconds- certainly significantly different to the usual approach release.
The other thing is that it is vrtually impossible to see "what is what" in the centre of your chart. It is as bad as attempting to follow the tracks on a PCB design- it certainly does not show clearly the aspect sequence as an overall picture. Personally I do not like vertical lines as can't tell at a glance which way the thing reads as converging / diverging- it is not too bad when simple and well separated as it is beyond 121 etc., but in this area the convergence of 109 and 111 gets rather mixed up with the divergence beyond 111- it isn't immediately evident whether 109 does in fact read to 127 for example. I know it is difficult in exam conditions but you'd lose some marks- its a bit of evidence to the examiners is that you hadn't planned before getting stuck-in; ideally for example the horizontal lines from 111 would go to 123 rather than 125 but that would mean that more space should have been left at the top and less at the bottom of the sheet.
One tip that can help in such areas is you could have converged the "similar sequence" from 123/125/127 into one set of 3 lines, just keeping the pair of lines from 121 to converge closer to the destination signal 147. As well as being clearer it would have saved some width and thus given more horizontal room in the centre part of the diagram. [See next post]
Overall though not bad; I've seen far worse. If you'd got the more restrictive release time for the Warning route I reckon that it might have scored a credit
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When looking for something else I chanced upon the attached which was my attempt at 2004 I did several years ago. It illustrates how it is possible to make the presentation rather clearer by spacing it out appropriately, joining the similar sequences together and using diagonal lines that helps particularly when a 3 apect sequence converges with a 4 aspect sequence to the same exit signal.
PJW
