2012 Mod 2 calculations and relevant signalling of layout

[DLMC]

I would like to ask two questions about 2012 calculation

1) In question 1b signalling system determination, the calculation of 3 and 4 aspect headway is around 120 seconds and 90 seconds respectively. How shall I determine the choice of signalling system? Shall I base on the number of trains per hour between D and H? i.e. 5 trains per hour and so 3 aspects is a cost effective solution to fulfill the headway requirement.

2) for the re-entry time, is it mean the stopping headway plus the 5 minutes of layover time at B?

Thanks

[PJW]

1) In essence, yes.
Your headway calcs are (I guess) based on all trains non-stopping travelling at 120km/h, but of course if you consider those 5 trains then 2 of them stop and 1 is a freight (with a lower maximum speed of 100km/h). Hence whereas 2 of the trains will occupy one train path each, the other 3 will eat up a bit more capacity than you might oversimplistically be asssuming.

It may well be that 3 aspects will still be enough but you should make sure that there is a fair bit of contingency over and above the 5 trains per hour.

Thinking about the stopping service, if you do a crude estimate and state that for every station the stopping train will lose say 90 seconds (in slowing down, at reast and re-accelerating) compared to the through service then that's nearly 5 minutes extra overall. Hence if your signalling only provided for a non-stop headway of 12 minutes (in order just to achieve 5 non-stop trains per hour) then your two stopping train would actually be sing up broadly three such train paths between them and the freight wouldn't be able to run as it too will use more than one train path as it travels at a slower speed.

If however you provided signalling to give say a 8 minute design headway it sounds much more acceptable and you could envisage:
a fast train at 00
a freight train at 09
a stopping train at 18
a fast train at 30
a stopping train at 48
this clearly has plenty of capacity and doesn't over restrict the timetable planners who have to juggle different constraints like these all around the railway.

I haven't done the sums but merely guessed, but assuming that is all the capacity we want,then it may be that we do not even need continuous 3-aspect signalling; isolated 3-aspect (i.e. stop and distant 2 aspect signals) may well be adequate.

However do not be a slave only to headway; think where you need signals to protect hazards such as junctions and to be operable (so reasonably close to pointwork, ability to have the routes needed).
You are bound to need signals at the end of the platforms at E and you'll also want a signal only about 700m prior to that (in order to protect the station throat)- you'll need to figure out how that accords with the braking distance requirements and what to do about any incompatibiity between that and the optimum position from a junction protection perspective.

Similarly you have other more-or-less fixed signal positions at the end of platform 2 at G and on the Up Main approaching that junction from the right. Somehow you need to juggle things so that you can place signals where you want them at these sites and get an integral number of signal sections separating them, whilst abiding by the braking & aspect sequence considerations. Certainly you must make sure that the headway requirements are also achieved, but given these are not onerous, I suspect that the signalling will be more determined by the spacing of the layout features.

2) For the re-occupation time on the single line you need to calculate how long it takes train 1 from the time it leaves platform 2 at E to travel the line, accelerating, decelerating, dwelling at intermediate stations as required, then its 5 minute turn-round time at B and returning similarly until it gets clear of the long crossover (via the switch diamonds) into platform 1 thus enabling train 2 to enter from platform 2 in order to do similarly.

Quite a lot of calculations, but look at the plan carefully as the examiners have been kind to you and you can use some of the sums multiple times.
I think this was the bit of last year's paper "that separated the sheep from the goats".

Obviously I don't know what twist there will be in 2013, but you'd be wise to expet something different, rather than the overly formulaic apprach that was the case for many years previously.

I would like to ask two questions about 2012 calculation

1) In question 1b signalling system determination, the calculation of 3 and 4 aspect headway is around 120 seconds and 90 seconds respectively. How shall I determine the choice of signalling system? Shall I base on the number of trains per hour between D and H? i.e. 5 trains per hour and so 3 aspects is a cost effective solution to fulfill the headway requirement.

2) for the re-entry time, is it mean the stopping headway plus the 5 minutes of layover time at B?

Thanks

[Nagasri.Jonna]

Hi all,

Please review the calculations attached for 2012 mainline layout and provide your comments. Please tell me whether the approach I have followed is correct or not.



Regards,
Nagasri.J

[PJW]

Not ideally placed to review in detail right now (in hotel, laptop with small screen and also things I need to get done for tomorrow) but overall it looks reasonable and certainly clearly presented.
I did notice that you simply quoted the headway formulae without explaining their derivation.
I'll review over this coming weekend in more detail. However on the last page you state 3 aspect signalling; given that from memory on the 2012 paper the discussion is about the traffic on the single line, I believe that this would not be continuous 3 aspect MAS but a few red/green signals each with their own yellow/ green distant (which is called isolated 3 aspect signalling) The headway formula in that case is independent of braking distance as it relates only to the positioning of the stop signals.

Hi all,

Please review the calculations attached for 2012 mainline layout and provide your comments. Please tell me whether the approach I have followed is correct or not.



Regards,
Nagasri.J

[PJW]

The first part of the question asked you to determine minimum braking distances for the permissible speeds for the traffic specified on the layout.
You did this for 140km/h and 100km/h so that covers the Main and Slow & Branch and showed your workings so that was fine for the passenger but perhaps you should have done the 40km/h for the freight only portion of line as well, but you would barely be losing any marks by omitting.

The second part of the question asked you to calculate, for both 3 & 4 aspect signals spaced at their minimum spacing, the best theoretical headway for D - H and explain how these calculation determine choice of signalling.
You did the calculations ok BUT did not explain where the formulae you used came from. Hence you'd be losing marks; an annotated diagram would have gone a long way towards this.

You made no attempt to explain how you would use the numbers you obtained, in conjunction with layout constraints, to determine where to provide what form of signalling. Hence you'd have thrown away all the relevant marks for this portion.

The last part of the question asked you to calculate how soon after one train entering the branch at station E that it would be before a second train could do so.
You answer does not immediately seem to be answering the question, but doing some preliminary calculations that will be needed; looking ahead to your last page means that the examiner can at least see where you are heading from the diagrams there. However the title "stopping headway" and the comparison between a non-stop and a stopping train does not immediately seem helpful. Having said that, the idea of some of the things you are calculating is right, but the numbers are wrong!

You used 38.89 m/s which is the maximum permissible speed on the mainline; firstly you should have used the headway timetabled speed and secondly this part of the question relates to the branch.

You should have used 27.78 m/s and thus at 0.5m/s/s it would take 56 secs to accelerate or brake. The average speed would be 13.89 m/s and therefore train would cover 778m.

In this question you should not have been comparing with a non-stop train; what you should have done is to use these figures to out numbers on that diagram on your last page. The third page really isn't useful for answering the question that you were asked.

For the purposes of the calculation of the branch reoccupation time, there is only one train on the branch at any one time. You are looking to calculate how long it takes a train to accelerates from E, coast at steady speed, brake to stop at C, dwell, accelerate, coast and brake again, then lay-over at B before doing similarly on the return trip and clear the branch ready for the next train to do similarly.

If you look carefully at the layout diagram, you'll see the examiners have been kind in that the inter-station distances are the same so once you have calculated the E to C time then that is the C to B time and therefore for the entire journey then you multiple the time by four.

So you only now have to calculates the distance over which the train will be travelling at a constant speed- i.e. the full inter-station distance minus the 778m taken to accelerate and another 778m taken to brake. The rest is covered at 27.78 m/s and hence you can easily find what that time is and add it to the 56s for acceleration and another 56s to brake to get the running time that is then multiplied by four. Add in the two dwell times at C, then the layover time at B and make some sort of allowance between one train leaving the branch at E and the next one actually entering and you have the requested number.

Therefore although you calculations were broadly ok and were components of the answer, I don't think you really understood what was needed to answer the question that was asked in 2012- have a look at the earlier attempt and my comments upon it in this thread.
You'd have got some credit of course for showing some understanding of the basic calculations, but not the majority of marks for this section.

Not ideally placed to review in detail right now

Hi all,

Please review the calculations attached for 2012 mainline layout and provide your comments. Please tell me whether the approach I have followed is correct or not.



Regards,
Nagasri.J