(26-09-2011, 10:09 PM)interesting_signal Wrote: I'm struggling with part 2 (iii) of this question which asks to "Describe the potential impact on the strike in distance of a change to a cab signalling system". (This is for a crossing with four barriers which close the road completely, and barriers close automatically).
It's only worth 2 marks in the question so don't need to say too much, but I really don't know much on this topic. Is information such as strike in points easily updated in cab signalling systems?
Thanks.
It is a it of an odd question. I think your answer could include:
1. Assumption whether there is lineside train detection. If not then the level crossing would have to be triggered by the position reports from the trains themselves and therefore how frequently they are polled, the likelihood of communication drop out and the degree of certainty with which they know their position could all factor. Certainly wouldn't be comfortable with operating an AHBC like this but in the case of CCTV if the LX not initiated soon enough then the train will get delayed rather than be unsafe.
2. Similarly the time delay between the level crossing having achieved down detection with crossing clear,via the interlocking, the RBC, the EVC (assuming ETCS) before the MA extension is visible to the driver and the ATP is relevant.
So I think it is more to do with unintentional system latency than anything else. Don't forget, if you want something implemented quickly, use relays and the speed of light between the signal and the driver's eye rather than many separate processors that may be changing 0s and 1s at the rate of GHz yet takes an age to do anything macroscopic!
PJW