13-06-2010, 11:05 PM
(05-06-2010, 01:25 AM)PJW Wrote: It wasn't that your calculations were wrong or even that you calculated completely the wrong thing; it was that you didn't see the signalling significance and therefore do the whole job.
So to get into this, then I suggest you do the same sort of thing as I did but with a string of 8, 3-aspect signals at 2km spacing with a platform in the middle of the diagram- hence you will be halving both my distance and time scales I suspect.
Don't worry about drawing a pretty curve or its shape- just plot the two you really need re
i) where the front of the train reaches its constant speed
ii) where the back of the train clears the various overlaps
Remember that the train remains a constant length and travels at the same speed as the front (ignore both the effect of couplings and general relativity!)
I know that you won't have time to do all that in the exam itself, but by doing it you'll appreciate what you are trying to do and therefore be able to distill the essence to present in the exam.
OK, I've spent a bit more time on the stopping calcs and have had a go at drawing the graph for 3-aspect signalling, as per previous my calculations. Apologies in advance that it's a bit messy! Also, had to squash in signal 5, so that part's not quite to scale.
My conclusion was that 3 aspect signalling can meet the stopping headway requirement of 360s. My graphs shows that a headway of 242s can be achieved with this spacing.
Note, the aspect sequence only relates to (is caused by) the first train. I had added these on before I drew the second train. Was going to scribble them out, but concluded it might be less messy to leave them in. Anyways, I've got a better understanding of this now, but need a lot more practice.