2005 Module 2 Layout Calculations

[Peter]

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.

Your presentation of the non-stopping headway is very concise and quite clearly presented.

For stopping, you have made an assumption that will not always be valid - ie that there is a starter signal near the end of the platform. In so doing, you have "saved" yourself some of the time that is wasted when the train is accelerating. If this was not the case and the train had to travel sufficiently far that it gets back to line speed, this part would be longer and the time at line speed slightly less, so you would find you are a bit nearer the mark (I think in this case you would be OK, but you have not proved this). Check that the numbers work if the station and the acceleration come just after the signal.

Your table presentation is good, but some of your headings are misleading. You have called section b "distance between signals" when in reality it is "distance between signals minus the distance it takes to decelerate".

Other than that, you seem to have done it well.

Peter

[alexgoei]

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.

Your presentation of the non-stopping headway is very concise and quite clearly presented.

For stopping, you have made an assumption that will not always be valid - ie that there is a starter signal near the end of the platform. In so doing, you have "saved" yourself some of the time that is wasted when the train is accelerating. If this was not the case and the train had to travel sufficiently far that it gets back to line speed, this part would be longer and the time at line speed slightly less, so you would find you are a bit nearer the mark (I think in this case you would be OK, but you have not proved this). Check that the numbers work if the station and the acceleration come just after the signal.

Your table presentation is good, but some of your headings are misleading. You have called section b "distance between signals" when in reality it is "distance between signals minus the distance it takes to decelerate".

Other than that, you seem to have done it well.

Peter

Hello Peter,

Some further points to clarify please:

1 In the IRSE Study Pack (the IRSE Support Materials), there is a power point presentation on Calculations. I noticed that when using the Train Length to compute the Non-Stopping Headway, the longer train length value was used even though the permitted speed value (in this case of the Freight Train) was slower. Should we therefore be always using the longer train length?

2 Your point about saving some time when the train is accelerating to reach full line speed - So would it correct to say that to determine if 3 aspect signalling could meet the stopping requirement headway of 218 secs (with 10% contingency) it would be necessary for completeness to incorporate a further 15 secs which is the time taken in order to reach the full line speed of 100 km/h? If the answer is yes, for this particular layout and with 10% contingency, the headway time between 2 trains both travelling at 100 km/h would be 226 secs. And therefore the conclusion that 3 aspect signalling would not have been able to meet the stopping headway requirements.

One more point to add for all taking the exams - in the 2009 paper the requirements are very clear - no contingency in the headway computation. Other students may wish to take read and take note of this should it appear in the exams.

Look forward to your reply

Cheers

[PJW]

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.

Your presentation of the non-stopping headway is very concise and quite clearly presented.

For stopping, you have made an assumption that will not always be valid - ie that there is a starter signal near the end of the platform. In so doing, you have "saved" yourself some of the time that is wasted when the train is accelerating. If this was not the case and the train had to travel sufficiently far that it gets back to line speed, this part would be longer and the time at line speed slightly less, so you would find you are a bit nearer the mark (I think in this case you would be OK, but you have not proved this). Check that the numbers work if the station and the acceleration come just after the signal.

Your table presentation is good, but some of your headings are misleading. You have called section b "distance between signals" when in reality it is "distance between signals minus the distance it takes to decelerate".

Other than that, you seem to have done it well.

Peter

Hello Peter,

Some further points to clarify please:

1 In the IRSE Study Pack (the IRSE Support Materials), there is a power point presentation on Calculations. I noticed that when using the Train Length to compute the Non-Stopping Headway, the longer train length value was used even though the permitted speed value (in this case of the Freight Train) was slower. Should we therefore be always using the longer train length?

2 Your point about saving some time when the train is accelerating to reach full line speed - So would it correct to say that to determine if 3 aspect signalling could meet the stopping requirement headway of 218 secs (with 10% contingency) it would be necessary for completeness to incorporate a further 15 secs which is the time taken in order to reach the full line speed of 100 km/h? If the answer is yes, for this particular layout and with 10% contingency, the headway time between 2 trains both travelling at 100 km/h would be 226 secs. And therefore the conclusion that 3 aspect signalling would not have been able to meet the stopping headway requirements.

One more point to add for all taking the exams - in the 2009 paper the requirements are very clear - no contingency in the headway computation. Other students may wish to take read and take note of this should it appear in the exams.

Look forward to your reply

Cheers

1. Haven't actually got to hand what you are looking at, but if it is as you say then I'll hold my hands up to that. The length of a train that can't travel at the speed for which the headway is calculated is irrelevant. Well spotted.

2. I'll leave the specifics of this to the other Peter. However I suggest you look at the graphs on attachments at end of this thread to get a better feel for what is the limiting section and sequence.

3.I'll admit to that as well. A certain examiner criticised students at an exam review for the way they interpreted the former wording. I criticised him privately later for doing that, given that there was actually significant ambiguity. I then got the job of revising the wording; note that I did several versions depending on the scenario, so no guarantee that you'll get the same version this year, but I hope whichever is used that it will be equally clear what is required.

[Peter]

2 Your point about saving some time when the train is accelerating to reach full line speed - So would it correct to say that to determine if 3 aspect signalling could meet the stopping requirement headway of 218 secs (with 10% contingency) it would be necessary for completeness to incorporate a further 15 secs which is the time taken in order to reach the full line speed of 100 km/h? If the answer is yes, for this particular layout and with 10% contingency, the headway time between 2 trains both travelling at 100 km/h would be 226 secs. And therefore the conclusion that 3 aspect signalling would not have been able to meet the stopping headway requirements.

Yes and no. As I said in the earlier post, the extra distance that you have to travel to complete the acceleration, you must take off of the distance (and hence time) that you travel at line speed. So while you have to add on 15 seconds for the acceleration, you take off 13 seconds for the 364m that are now not being travelled at linespeed.

In this case it does not make a lot of differnece to the outcome - a change of 2 secs, so your assertion that 3 aspect works was correct and it may seem pedantic, but getting to the right answer for the wrong reason will do you no favours if it could lead to other errors in the future.

I hope I have managed to explain that OK.

Peter

[NJK]

Dear Members,

I have attempted module 2 2005 layout calculation. Kindly review it and give your comments for my further enhancement.

Thanks in advance.

Regards,
NJK

[kiran218]

Hi NJK,

10% of contigency is required for headway time. Under what conditions we have to fallow this?

With Regards,
KiranKumar

Dear Members,

I have attempted module 2 2005 layout calculation. Kindly review it and give your comments for my further enhancement.

Thanks in advance.

Regards,
NJK

[PJW]

Some allownce for contingency is usually sensible unless it is evident that the specified headway is very much less than would ever be needed to make some allowance for perturbation in a timetable.
The more the train service approaches that of a metro to be used to maximum capacity for an extended period of time, the greater the allowance which should be made; where there are a few spasmodic trains within the hour then no significant margin between the "timetabled need" and the "signalling design" is needed- even so it is sensible to hae a small allowance to give a small margin. f you were signalling contractor and designed signalling precisely to deliver say 180second headway, yet because of slight rounding errors and slightly false assumptions re how trains are driven etc it only delivers 182secs, then the railway company could state you had failed to meet an essential criteria and to modify would be extremely expensive and take a lot of time and effort, If only the design had been for 175sec, then if it turned out that it really only delivered 178sec then still within the threshold.

The whole point is that it is a judgement call, depending on all the circumstances; for IRSE exam I would not be overly concerned whether put a 10 second contingency or any reasonable percentage say from 2 - 20 per cent. Undderstand WHY we put a margin and the factors that determine what might be appropriate; don't worry re the absolute value in any particular circumstance!

Hi NJK,

10% of contigency is required for headway time. Under what conditions we have to fallow this?

With Regards,
KiranKumar

Dear Members,

I have attempted module 2 2005 layout calculation. Kindly review it and give your comments for my further enhancement.

Thanks in advance.

Regards,
NJK

[PJW]

Some allowance for contingency is usually sensible unless it is evident that the specified headway is very much less than would ever be needed to make some allowance for perturbation in a timetable.

The more the train service approaches that of a metro to be used to maximum capacity for an extended period of time, the greater the allowance which should be made.

Where there are a few spasmodic trains within the hour then no significant margin between the "timetabled need" and the "signalling design" is needed- even so it is sensible to have a small allowance to give a small margin. If you were the signalling contractor and designed signalling precisely to deliver say 180second headway as per the specification, yet (because of slight rounding errors and slightly false assumptions re how trains are driven etc) it turns out that it only delivers 182secs, then you have a big problem. The railway company would state you had failed to meet an essential criteria; to modify the signalling would be extremely expensive and take a lot of time and effort. If only the design had been for 175sec instead, then if it turned out that it really only delivered 178sec there would have been no issus since still within the threshold. The marginal cost of slight exra capacity at the initial stage would have been quite trivial.

The whole point is that it is a judgement call, depending on all the circumstances; for IRSE exam I would not be overly concerned whether put a 10 second contingency or any reasonable percentage say from 2 - 20 per cent. Understand WHY we put a margin and the factors that determine what might be appropriate; don't worry re the ABSOLUTE VALUE in any particular circumstance!

Hi NJK,

1o percent of contigency is required for headway time. Under what conditions we have to fallow this?

With Regards,
KiranKumar

Dear Members,

I have attempted module 2 2005 layout calculation. Kindly review it and give your comments for my further enhancement.

Thanks in advance.

Regards,
NJK

[kiran218]

Dear PJW,

If contingency is provided for headway is 10% and headway time is 150sec. Practical headway time we have to take 150/1.1=136 or 150-10%=135 which is we have to fallow in the exams. what is the difference between two.

with regards,
kirankumar

Some allowance for contingency is usually sensible unless it is evident that the specified headway is very much less than would ever be needed to make some allowance for perturbation in a timetable.

The more the train service approaches that of a metro to be used to maximum capacity for an extended period of time, the greater the allowance which should be made.

Where there are a few spasmodic trains within the hour then no significant margin between the "timetabled need" and the "signalling design" is needed- even so it is sensible to have a small allowance to give a small margin. If you were the signalling contractor and designed signalling precisely to deliver say 180second headway as per the specification, yet (because of slight rounding errors and slightly false assumptions re how trains are driven etc) it turns out that it only delivers 182secs, then you have a big problem. The railway company would state you had failed to meet an essential criteria; to modify the signalling would be extremely expensive and take a lot of time and effort. If only the design had been for 175sec instead, then if it turned out that it really only delivered 178sec there would have been no issus since still within the threshold. The marginal cost of slight exra capacity at the initial stage would have been quite trivial.

The whole point is that it is a judgement call, depending on all the circumstances; for IRSE exam I would not be overly concerned whether put a 10 second contingency or any reasonable percentage say from 2 - 20 per cent. Understand WHY we put a margin and the factors that determine what might be appropriate; don't worry re the ABSOLUTE VALUE in any particular circumstance!

Hi NJK,

1o percent of contigency is required for headway time. Under what conditions we have to fallow this?

With Regards,
KiranKumar

Dear Members,

I have attempted module 2 2005 layout calculation. Kindly review it and give your comments for my further enhancement.

Thanks in advance.

Regards,
NJK

[PJW]

It really doesn't matter. The 10% figure is "plucked out of the air" and so, whereas I agree there is a small amount of ambiguity, any discussion whether the calculation is based on 136s or 135s is irrelevant- personally I'd do the division and therefore utilise 136s, but IT DOESN'T MATTER.
You have the concept- now move on.

Dear PJW,

If contingency is provided for headway is 10% and headway time is 150sec. Practical headway time we have to take 150/1.1=136 or 150-10%=135 which is we have to fallow in the exams. what is the difference between two.

with regards,
kirankumar

The whole point is that it is a judgement call, depending on all the circumstances; for IRSE exam I would not be overly concerned whether put a 10 second contingency or any reasonable percentage say from 2 - 20 per cent. Understand WHY we put a margin and the factors that determine what might be appropriate; don't worry re the ABSOLUTE VALUE in any particular circumstance!

Hi NJK,

1o percent of contigency is required for headway time. Under what conditions we have to fallow this?

With Regards,
KiranKumar

Dear Members,

I have attempted module 2 2005 layout calculation. Kindly review it and give your comments for my further enhancement.

Thanks in advance.

Regards,
NJK