(04-10-2012, 10:48 AM)ricky Wrote: I approached this by drawing a diagram which showed the crossing the 3 signals in approach and the overlap of the final signal before the crossing. I assume that for minimal delays a train driver would only encounter green aspects.
I am a little unsure of braking distances and signal sighting distances so I assumed 800m to stop from 140km/h and 200m to see a signal. This meant that the two signals closest to the crossing were 800m apart. This plus the 180m overlap plus an extra 10m as the overlap wouldn't want to be in the crossing area means the final signal must display a green aspect when the train is 1190m from the crossing.
Methodolgy seems sound but distances rather low. 800m sounds to be rather like braking of a 9percent g train from around 75mph and thus rather an understimate for 140km/h. Also sighting distance at that speed would probablly be more like 300-400m, but the distances are not rediculous and it is the principle that the question is after, so good enough for that.
Quote:The crossing sequence should show amber for 3 secs before red lights for 6s before the entrance barriers lower. Then 8s for the barriers to lower and the signaller to confirm the crossing was clear before the exit barriers lower which shall take a further 6seconds. This totals 23secs. 23secs at 140km/h is 894m so the strike-in should be 2084m from the crossing.
OK except that the signalman gives crossing clear after ALL the barriers are down; you probably need to allow at least another 5 seconds for their attention to be drawn, finish the task in which engaged (i.e. stop reading the newspaper- I meant their Rule Book!), look carefully at the CCTV monitor and then push and release the CC button.
Quote:i) I don't think 4 aspect would affect this distance as the driver is only seeing green aspects but the three signals closest to the crossing will be 800m apart (2x400m) so the strike in is the same
The crossing protecting signal starts at red and for 3 aspects the on which changes from Green is 2 signal spacings (each a minimum of braking distance) away.
For 4 aspect the one which changes from Green is 3 signal spacings (each a minimum of half braking distance) away.
Therefore not quite the same.
Also if get crossing down slightly too late so that driver gets cautionary aspect the train will be realy stopped by the time it gets to see the signal ahead is no longer at red; on 4 aspect signalling then the driver will get an update at the signal they had to expect to be single yellow but by then may well have cleared and thus can stp braking and reaccelerate befor so much speed had been lost.
Quote:ii) this may save a couple of seconds @ 140km/h is 78m closer to the crossing.
iii) the sighting distance will no longer be required so 200m closer to the crossing.
On the final part I was really unsure so if anyone has anything they can suggest on this I would appreciate it. The only thing I know about in this respect are level crossing predictor technology that uses the change in impedance to calculate the speed of the train and whether it is deccelarating. This could be used with a treadle for stopping trains but i am not sure of the traditional meathods.
Thanks for reading
PJW