2010 Mod 2 layout Calculations

[yuvaraj.narasimmalu]

Hi there,
Please sugest some valuable comments on the attached headway calculations for IRSE Modul2 2010 question.

Thanks and regards,
Yuvaraj

[PJW]

Obviously the fact that it is typed and includes the question are good for learning and posting here, but not representative of what you'd do in the exam itself. It is certainly worth also practising in as near as exam conditions as possible.

That said it is convenient that you have included here and definitely a good sign that you took note of what the question paper itself asked for, in addition to taking the relevant numbers from the layout.

I think that in general you struck a good balance between explanation and brevity. I think I'd have shown how kph converted to m/s but otherwise I think part a was perfect; you correctly worked out for the branch as well as the mainline.

Part b(i) was also very good; enough to explain but nothing irrelevant or repetitive. Your diagram was great, but you should have said that d= signal spacing. Obviously for this actual question it wanted d=B, but no harm in showing how you have since likely to need to consider wider spacing when determining appropriate signalling for the particular layout.

You were right to comment that the headway was well within the specification; in fact it is so well within that you should (either here or probably better at the end) have stated that provision of continuous 3 aspect signalling would be excessive for the traffic demand on the G to H portion of the layout.

Stopping headway. I quite liked your diagram but it would have been improved to have labelled vertical axis as speed and horizontal as distance.

I did not like the assumption that the driver would ignore a yellow aspect and continue at their normal speed until then applying a full brake application to come to a halt in the station. However a least your diagram and calculations matched; also the question did ask for best possible headway and hence if you had pointed out that you had made this assumption of unrealistic driving in order to derive the absolute limiting value, then I'd have felt more inclined to have accepted.

So all your calculations are ok until you add them together to get what you cal the headway time. You do seem to recognise, because you then state the calculation is not accurate; what I don't understand is why you didn't therefore correct!

You have basically calculated the time it takes train 1 from passing the platform protecting signal to
a) run at constant speed until gets to last minute braking point,
b) brake
c) dwell
d) accelerate to clear the overlap
You are correct that this is the earliest time that train 2 can reach the sighting point of the 2nd signal in rear which will just change to green.
However this train is moving at headway speed whereas train 1 is not since it is still accelerating. Therefore if look at the situation when train 2 reaches sighting point of the platform protecting signal it will be showing yellow (assuming equal signal spacing then train 1 cannot possibly have cleared the section as travelling, initially at least, far slower than train 2) Therefore not a chance; it is a certainty.
It may be that the driver may just see that signal change to Green before they pass it; if not the brakes will be on and the fast train will be getting ready to stop, until the next signal comes into view which almost certainly is by that time Green and train 2 needs to accelerate again- hence it has been affected by train 1.

It therefore makes no sense to specify the size of a margin, because you haven't even demonstrated that the requirement can be met (even with very aggressive driving as assumed earlier).

What you should have done was to have calculated how long it would take for train1 to reach headway speed; you have then got back to a steady state and the relative separation of the two trains does have significance.

Given this, your last portion would probably need adjustment but was otherwise on the right lines.
Your biggest mistake though was just to give one solution for the layout as a whole.
The stopping calculations applies between A & G; this is obviously far more onerous than the non-stop.
Between G & H far less signalling is needed; therefore should have opted for isolated 3 aspect signalling with Red/ Green signals placed as needed to protect junctions etc but at least enough to deliver the headway (but 10 minutes at 140km/h is quite a long way) and a Yellow / Green at braking distance on the approach to each.
Similarly there is not much freight on the branch and the speed is low; need to consider what sort of signalling to place here.

I can't remember an IRSE Mainline layout for which "one size fits all".

The 2010 layout was obviously intended to "find out" those students who can't be bothered to do the stopping headway calculations and hope to "get by" just by doing the non stop. Another way of doing that is to give a layout where the main station is a terminal and therefore all trains have to stop / start from rest. It was quite a few years ago now that such a layout featured, and if I were on the exam committee I'd have been arguing that it was high time that it made its return. I've said this for a couple of years now; either I am well out of tune or one day it will pop up and I just wonder if 2011 is going to be that year......

PJW

Hi there,
Please sugest some valuable comments on the attached headway calculations for IRSE Modul2 2010 question.

Thanks and regards,
Yuvaraj

[vedprakash]

Hi

I did calculation for 2010 layout but I lost the plot somewhere, please have a look and bail me out.

Ved

[PJW]

I think that most of it is very good.
A few quibbles:
a) I think that 1000/(60 x 60) is a clearer way of showing the conversion of km/h imto m/s than 5/18
b) Having decided to round up the speed to 2 significant figures, you shouldn't quote to the precision you have when you square to get the distance.

Must admit that I think question wording is poor regarding whether need to show best headway for both 3 aspects and 4 aspects. One could say the best implies 4 aspects, hence don't worry re wasting time on 3 aspects; alternatively one could read it as signals at braking distance and therefore it is the 4-aspects calculation which is not needed. Therefore I think like you I'd have done both to be sure.


You didn't quite do the stopping headway bit, although most of what you did was ok and going in the correct direction. The issues that I have with it:

1. You have assumed that driver does not brake at all when passing the signal prior to the station but continues at headway speed until last possible moment to brake to a stand. This would not be how a driver would be expected to drive in the UK if that signal had been showing a cautionary aspect. It is not clear from your explanation what aspect you are expecting that driver to have received.

2. What you have actually calculated is the TOTAL time for a stopping train to travel the distance of two signal sections (188.2 sec). You then add this to the non-stop headway time without any explanation relating to how this gives the result you were asked for; you are supposed to be calculating the timetabled allowance between train 1 and train 2 when leaving their origin such that after train 1 has stopped and train 2 has not that they are then at minimum headway. I think you have nearly done what was needed, but not quite.

3. You have done the maths purely on the basis of decelerating and accelerating of trains, yet really have not related this to signalling at all. We need to be sure that the signalling we provide does not prevent the trains being driven in such a manner that the required headway can be achieved; your aswer does not address this element as far as I can see.

4. You haven't done much to explain how the results determines your choice of signalling; perhaps you intend to do that determination next, but don't forget to explain it to the examiner- it is part of the question so I guess there may be some 4 or 5 marks reserved for it.

Overall thought was neat, clear, well presented, suitable level of detail so a very respectable effort. It is the stopping calcs which demand more attention; see also my comments on a recent attempt at 2011 in which the same fundamental question is being addressed.

Hi

I did calculation for 2010 layout but I lost the plot somewhere, please have a look and bail me out.

Ved

[vedprakash]

Thank you for your feedback, I can connect things now, need to do lot of homework, will come back to you soon.