2005 Module 2 Layout Calculations

[PJW]

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".