29-05-2010, 09:13 AM
(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)