Signal approaching the station

[mark bedford]

I should have raised this question before the exam, I hope it is useful to future candidates.

If a signal approaching the station has a call on route then by GKRT0044 it is limited to being positioned no more than 400m from the platform. If we now consider that the caution in a 4 aspect sequence should be no less than 1/3 braking distance from the red, then it could be the call on signal that is the limiting factor. I think approx 125km/h equates to 400m=1/3 braking distance. Often stations have speed limits which tolerate the 400m limit, most of the IRSE papers I've practiced on do not have speed limits. I assume there are two options, one to apply a speed limit to the station (but that is outside the requirements of operation) or to control the aspect sequence as per closing up signals. This would then affect the stopping calcs. Although I had considered this during my studies I forgot it during the exam and placed my approach signal around 600-700m from the platform.

Please comment if you agree or if I've overlooked something
Mark

[PJW]

I should have raised this question before the exam, I hope it is useful to future candidates.

If a signal approaching the station has a call on route then by GKRT0044 it is limited to being positioned no more than 400m from the platform. If we now consider that the caution in a 4 aspect sequence should be no less than 1/3 braking distance from the red, then it could be the call on signal that is the limiting factor. I think approx 125km/h equates to 400m=1/3 braking distance. Often stations have speed limits which tolerate the 400m limit, most of the IRSE papers I've practiced on do not have speed limits. I assume there are two options, one to apply a speed limit to the station (but that is outside the requirements of operation) or to control the aspect sequence as per closing up signals. This would then affect the stopping calcs. Although I had considered this during my studies I forgot it during the exam and placed my approach signal around 600-700m from the platform.

Please comment if you agree or if I've overlooked something
Mark

1. Sometimes in reality 400m for PL is not realistically achievable; there are derogations up to at least 600m for various specific sites I believe; you could have placed a note to this effect.

2. The one third / two thirds rule is for ACTUAL SIGNAL SPACING between YY and R; this may be significantly (say 30%) more than Braking Distance. There are instances where a site derogation against this is applicable; indeed if you think of means of transition re 3 aspects then in some circumstances it is almost inevitable.

3. Speed restrictions in the vicinity do help; it can be an option to propose in IRSE Exam provided has little / minimal impact of the train services specified to operate.

4. Signalling as closing up signals would certainly have headway impact as you say.

I've said before that layout design is a BALANCE; in reality signal sighting can be a huge constraint. In addition there are P'Way and other discipline considerations to put into the equation as well; certainly a non-trivial activity!! That's why it frequently takes years, not the 90 mins of the exam.

However for the exam, demonstratre that you a) know the rules, b) recognise which are sacrosanct and which can be bent, c) suggest mitigations when forced to do something that is in some respects sub-optimal and the examiner will recognise that this is a candidate actually knows what they are doing and isn't just following a painting-by-numbers recipe blindly and will mark accordingly.

PJW