IRSE Study Pack 090131 - Appendix G22 and 2004 Paper

[alexgoei]

Hello Peter,

I have been going through Appendix G22. I have two questions to clarify please:

On the second paragraph of page 269 where there was 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. Please see extracted page which is attached.

The other question I would like to clarify is that for this question can I take it that 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.

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

Thank you and look forward to your reply.

[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

[alexgoei]

Hello Peter,

Thank you for the reply and the clarifications.

Some more questions I am afraid:

1 How then would the reduction in overlap to 80 metres be determined? Is there any other standard other than GK/RT0064 that can be referred to? Or do we have to calculate it along the lines you mentioned in your earlier reply?

2 Specifically for the 2004 paper, the requirement for this paper would be to calculate if the non-stopping and non-stopping headway can be met. For the latter, to look at the section of the line where meeting this requirement is the most onerous. Am I correct? (although I am still trying to identify where this section is)

Thank you & Regards

[PJW]

1 How then would the reduction in overlap to 80 metres be determined? Is there any other standard other than GK/RT0064 that can be referred to? Or do we have to calculate it along the lines you mentioned in your earlier reply?

There is an unofficial spreadsheet but never published. In some quarters there is a reluctance to face up to controlling risk on the balance of probability. Everyone has always known that TPWS is not 100% train protection, it does far more than we could have reasonably hope for yet there are always those who wish to "guild the lily". A whole lot of politics between the Infrastructure Manager and the Train Operating Companies relating to "whose risk it is".........

Not really relevant from perspective of Mod 2 IRSE exam but a potential question in mod1, mod 3 or mod 7 perhaps.

Best I can do is refer you to a paper on TPWS I wrote some years ago which I have added as an attachment (was previously on main IRSE site following that YM conference but now removed I think to make way for more recent material).

Part 2 of your question I'll address later.

[PJW]

2 Specifically for the 2004 paper, the requirement for this paper would be to calculate if the non-stopping and non-stopping headway can be met. For the latter, to look at the section of the line where meeting this requirement is the most onerous. Am I correct? (although I am still trying to identify where this section is)

The actual question paper wording was:

Question 1
Determine theoretically, either by calculation or graphically, appropriate signal spacings for the braking characteristics and the intensity of traffic on offer. All calculations and graphs must be shown.

The layout paper asks for
a) "following fast" which is definitely meaning non-stop headway at 100k/h and
b) "following stopping" which I interpret as one stopping train following another stopping train- I think the wording isn't 100% unambiguous though so I'd state my interpretation on the paper.

So it is asking you to calculate what signal spacing (which also is compliant with the need for adequate braking yet not too excesssive) just satisfies the more onerous of these two requirements and then design the layout accordingly.

The whole point about non-stop headway on the main line is that it is constant speed, steady state so it is the same everywhere as speed of line is constant. Stopping headway pinch point is certainly station D because trains are stationary for 30 seconds and also be slowing prior and accelerating after. Also note that there is a stopping train service from A and from C that have converged by then.

So work out what signal spacing you need to satisfy the non-stop then put a few signals on the layout (you virtually have to have one at the exit of the platforms on both lines) and see if that delivers the required stopping headway.
Just taking a very quick look- through train 2.5min headway, dwell time 30 sec so have got 60sec for addditional time for stopping train re slowing down / speeding up. 100km/h around 30m/s and at brake rate of 0.5m/s/s takes 60sec to slow, similar to speed up. Comparing to a non-stop means that travellling at half speed so will lose 60sec compared to it. Hence obvious that the two constraints in this particular example are pretty comparable. Hence I'd put the signals before and after the station starter at minimum spacing and then do the sums to demonstrate would achieve the stopping headway requirement.
Does that make sense?

[alexgoei]

Hello Peter,

Thank you for the reply

Will digest and clarify further if required

Thanks again