IRSE Study Pack 090131 - Appendix G22 and 2004 Paper

[PJW]

some discussion on overlap length of which 225 metres was first mentioned and subsequently this was reduced to 80 metres, would it not be possible to reduce this further to 60 metres in line with GK/RT0064 in which at 40 kph (25 mph) the minimum overlap can be 60 metres? This could then be translated into a longer train dwell time at the station.

Potentially yes the standard does permit a shorter overlap than 80m; however overlaps should not be reduced by more than they really need to be. Be aware also that the standard tables of minimum overlap length versus speed have been around for years and the UK has become rather more risk adverse since Ladbroke Grove. Also now that we have TPWS (didn't exist when this table was first developed) there is a good chance of a train being stopped within a long overlap- but less in a short overlap. In particular the main risk at stations is "SASSPAD"- starting once platform tasks complete despite red aspect. It does take time to apply brakes on a train and even at low speed it'd be say 20m into the overlap before braking is applied. If you do the sums, even on a relatively optimistic basis, TPWS is not likely to stop the train within the length of a very short overlap particularly if it has any length of run-up if it starts mid platform; to me 80m is about as low as I'd want to go unless there was an overwhelming reason to compromise. In this case the only benefit of bringing down to 60m is a saving of 20m @ roughly 12.5m/s, so say 1.5 seconds. Perhaps on a Metro this would be critical, but on this layout...?

where the notes up front mention Permitted Speed 40km/h between certain sections of the line, this effectively becomes the permanent speed restriction for that section of the line instead of 100 km/h? However the braking distances along this section of the line where the permitted speed is 40 km/h, the distance between signals is still more than 700 metres (the braking distance) (I have the 1998 model answer) for safety reasons.

Yes the old talk was of "line speed" and Permanent Speed Restrictions where the speed along a particular portion was lower. Now with the number of differential speeds for different traffic, the terminology is Permissible Speed for a particular traffic on that portion but no defined "line speed" as a default. So yes the layout here has a low speed portion through the station area.

For the braking distance you have to consider the speed the train is permitted to do when sighting the caution signal. Obviously with a step-change, in reality a driver shouldn't be doing 100 when close to the 40 commencement, but we don't take that into account. I haven't looked at the answer in detail but yes I think the signal spacing could have been closed up; but you need to ask yourself would that have been beneficial. At 40kph concerns re over-braking are irrelevant. Also need to consider reasonable positions for signals and not provide more than required for the desired headway.

For the 2004 paper, under Required Headway, what does "following fast" mean - non stopping?

Yes. Headway is generally requested for
A) Fast train following Fast train at constant speed (but on this layout of couse even the Fast trains are slowing for the restriction and accelerating again)
and either
B) Fast Train (non-stop) following a Slow Train (that has stopped)
or
C) Slow Train following Slow Train (i.e. both on same service pattern stpping at station).

Hope that helps,
PJW