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Hello,
I have made an attempt at doing calculations for Module 2 layout paper of 2005. Please view the attachment and provide me with you feedback.

Regards,
Sid Shekhar
(31-07-2009, 02:47 PM)sidshekhar Wrote: [ -> ]Hello,
I have made an attempt at doing calculations for Module 2 layout paper of 2005. Please view the attachment and provide me with you feedback.

Regards,
Sid Shekhar

It'll be a little time before I get a chance to do this- I am pretty busy re the forthcoming mod2/3 event just at the moment. It may be that someone else will respond; otherwise I'll get around to it eventually.....
(03-08-2009, 08:01 PM)PJW Wrote: [ -> ]
(31-07-2009, 02:47 PM)sidshekhar Wrote: [ -> ]Hello,
I have made an attempt at doing calculations for Module 2 layout paper of 2005. Please view the attachment and provide me with you feedback.

Regards,
Sid Shekhar

It'll be a little time before I get a chance to do this- I am pretty busy re the forthcoming mod2/3 event just at the moment. It may be that someone else will respond; otherwise I'll get around to it eventually.....

Just got back from leave and had a look on the train coming home today.

There is a lot in here to show that you have a good basic understanding but there are several points where you have some flawed logic. You have also made some unexplained assumptions which may be fine if you get things right, but if the examiner cannot understand jumps that you have made which you have not explained, you'll lose out.

Your conversion of speeds to m/s at the beginning is always a good idea, but how many of them did you actually use? You may have wasted some time unnecessarily by converting every one and looking at the order of your table, you missed probably the most important one in this context and added it to the end. I am interested in the translation formula that you used because although most of your conversions are close enough, you have arrived at some interesting numbers. For instance, given a speed of 90km/h, you arrived at the very precise 24.30m/s whereas 90x1000/3600 in fact comes to the very round 25m/s, yet your 33.33m/s for 120km/h is spot on.

You have not explained what b is in your SBD equation. It is obviously the braking rate, but you should define all variables.

You stated that the "recommended" headway is 150s. 150s is a REQUIREMENT.

Why assume a sighting time of 10s? State the practice of the railway you are signalling for and give what their requirement is.

Your sums for the calculation of DGR are sound (using the value of 27m/s vice the actual 27.78m/s) the important correct thing here is that you used the value relating to the quoted headway speed of 100km/h.

You would do well to explain DGR, and the other abbreviations (S, O and L). They may well be familiar to someone form the UK, but where will the marker of your exam paper be from?

Again, the concept of N=DGR/SBD is commonly taught in the UK, but just to state formulae with no explanation and no demonstration of understanding will not score you highly. The statement that because N is 2.45 you will use 3 aspect with spacing 0.5DGR may be true, but you should explain what it means.

You did less well on demonstrating the understanding of the stopping headway. Your approach of breaking it down into sections is a good principle, but you have made a couple of unstated and incorrect assumptions.
1. that in t3 you will have travelled the full distance between the two signals and hence 2. when you start again to accelerate after the stop, the only distance you have to go is the overlap plus the train length. In fact, in what you have called t3, you started braking at the signal and took 54 secs to come to a stand. At a braking rate of 0.5m/s/s from 27.78m/s you will have travelled 729 of the 1361m between the signals. Also, when you have accelerated from rest after your (assumed) 30 sec dwell time, you have taken 39 secs to travel the 383m. You are in fact only travelling at 19m/s at this point, so your following train is still catching you up.

Do not assume that the station stop will coincide with a signal position. Based on the numbers you have used, irrespective of where the stop occurs, it will take 54 secs to stop during which time you will have travelled 729m, 30 s at rest and 54 secs to get back up to speed, again travelling 729m. This means that you have gone 1460m out of your Hd of 3375m. The other 1915m are at Hv (either before or after the deceleration / acceleration) and hence take 71 sec, thus your stopping headway time for one stop is 71+54+54+30=209 sec. This compares with 240 sec requirement so you were right that 3 aspect signalling at 1361m meets the requirement (with some contingency), but you did not show it correctly.

Hope that is all clear. Any questions, please come back.

Peter
Hello Peter,

I have two silly questions please:

1) In determining if 3 aspect or 4 aspect signalling is more suitable to meet the headway requirements for Stopping Trains, should the speed used be a) the permitted speed of 120 km/h for Passenger (All lines) or b) the following fast speed of 100 km/h which is for non-stopping trains.

2) Is contingency provision required for the Stopping Headway of 4 minutes which is 240 secs. With contingency provision of 25%, the technical headway would have been 192 secs.

The outcomes (ie. whether 3 aspect or 4 aspect signalling) are quite different depending which of the two variables is used.

Look forward to your reply.

Thank you and Regards
(29-05-2010, 04:33 AM)alexgoei Wrote: [ -> ]Hello Peter,

I have two silly questions please:

1) In determining if 3 aspect or 4 aspect signalling is more suitable to meet the headway requirements for Stopping Trains, should the speed used be a) the permitted speed of 120 km/h for Passenger (All lines) or b) the following fast speed of 100 km/h which is for non-stopping trains.

2) Is contingency provision required for the Stopping Headway of 4 minutes which is 240 secs. With contingency provision of 25%, the technical headway would have been 192 secs.

The outcomes (ie. whether 3 aspect or 4 aspect signalling) are quite different depending which of the two variables is used.

Look forward to your reply.

Thank you and Regards

There is no such thing as a silly question!

I do not think there is a perfect straight answer to the first question. On one hand, if you do not use the (lower) headway speed for the calculation, you will not have any ability to recover in any way. I see the point of your question though, that if you assume that the following, non stopping train is going at the lower headway speed, becasue it will, in may cases be doing the faster spped (near linespeed), then it will quickly get to a point that it is not running on greens, and hence is less than headway distance from the stopper.

The scenarios given in the exam obviously have their limitations and what effect the singalling in the area you cover in the question has on the next bit down the line is undefined. For example, whatever you put, if your fast train leaves your layout right on the limit of seeing a cautionary aspect following the stopper, that might not be good for a real scheme, but you will have met the requirements of the paper.

As for the second question, as with non stopping, in the real world, some contingency should be given otherwise any disruption will never have the opportunity to resolve itself. "How much?" is another question....

I guess the simple answer to your question is that there is no absolute right answer. Considering the relevant things that have an impact on the outcome and making some intelligent remarks about these (either as qualifications to a list of assumptions or in noted to go with the calcualtions) would allow the examiner to know that you understand the limitations of a) the scope of the question and b) the inevitable compromise that any signalling layout will be.

Peter (the other one)
Hello Peter (the other),

Thank you for your reply to my questions.

In addition to your point with respect to my first question, signalling the layout for stopping trains to proceed at the speed of 100 km/h when the permitted speed is 120 km/h does give drivers the chance to catch up on delays if they have to. So I agree that for Following Stopping at 4 minutes, the speed to use should be 100 km/h.

For my second question, I think it would be sensible to put in a contingency of 20% for the Stopping headway of 4 minutes giving a technical headway of 200 secs. Unless advised otherwise that there is no need for this, I intend to do so for the exams.

Thank you once again for setting up this very excellent forum! Without it, there is absolutely no chance for me to prepare for the exams half a world away!

Good night

Cheers
(29-05-2010, 04:23 PM)alexgoei Wrote: [ -> ]signalling the layout for stopping trains to proceed at the speed of 100 km/h when the permitted speed is 120 km/h does give drivers the chance to catch up on delays if they have to.

I think it would be sensible to put in a contingency of 20% for the Stopping headway of 4 minutes giving a technical headway of 200 secs. Unless advised otherwise that there is no need for this, I intend to do so for the exams.

It is certainly a good idea to show the examiners that you are aware of the difference between the "technical headway" on which the design is based from the "operational headway" which the trains are advertised to run. However I think that 20% is quite high and would be the sort of figure used for a dense service where the intention is to run trains throughout the whole hour at say 2 minute headway, not when there is an occasional need for trains 2 minutes apart but on average over the entire hour there are far fewer trains than this would imply. So this figure is more applicable to Metros than Mainlines and I'd go for something rather smaller; 10% makes the maths easy.....

You could (and indeed really did above) argue that no explicit contingency is needed at all because there is an implicit "service recovery" margin since trains may perhaps travel faster than timetabled to catch up after a delay. This does depend on whether that particular rolling stock as well as the track is capable of the higher speed of course. In this instance a student could include these as assumptions and argue that they therefore will not be including any explicit contingency allowance between operational and technical headway; this would be equally right.

Do be careful to read the question actually asked on the question paper itself; it used to be rather ambiguous but the examiners are now being more precise in what they require- and it may not always be the same. They might ask for the best possible headway that could be achieved with signals at minimum spacing, they might ask for proof that the signal spacing that you choose for your layout delivers the headway requirements, they might use the word pure to signify that you shouldn't make any contingency allowance, they might use the word practical to signify that you should...... If they do not make it clear to you, then you would be wanting to ask them a question to make it clear and unambiguous- this is impossible in the exam so state your assumption but do make sure that you have taken the whole paper into account (wording, train service pattern as well as stated headway requirements to achieve) rather than going into the exam with any precise set idea of what you'll definitely do. Go in with the mindset "I need to use the information presented to determine what, if any, contingency I should provide and I then need to explain to the examiners thatm I have used it to come up with a rational decision".
Hello PJW,

Thank you for your reply.

So for the exams, I will put in a contingency of 20% on the Non-Stopping operational headway and and 10% on the Stopping operational headway to derive the headway distance.

Point noted about reading the question and understanding the examiners requirements.

Will submit by 2nd June my 2005 Calculations taking into account all that has been mentioned in this posting.

Cheers
(01-06-2010, 01:32 AM)alexgoei Wrote: [ -> ]Hello PJW,

Thank you for your reply.

So for the exams, I will put in a contingency of 20% on the Non-Stopping operational headway and and 10% on the Stopping operational headway to derive the headway distance.

Point noted about reading the question and understanding the examiners requirements.

Will submit by 2nd June my 2005 Calculations taking into account all that has been mentioned in this posting.

Cheers

I was meaning that you shouldn't go into the exam with any pre-conceived values but make your mind up having asessed how many trains per hour are to use the layout and any "hidden" contingency; however if you want a number then I think that 10% in each case would be a better default.
Hello PJW,

Thanks. I understand your point about the contingency in deriving the technical headway being dependent on what the exam requires. Appreciate it.

I would also like to submit my calculations for the 2005 Layout which I hope you and other participants to this forum may wish to comment.

Thank you again and look forward to your reply.
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