Your explaination is simple and easier for me, haha. I think I have got what you said. But maybe I will try on another year's paper and post my answer here, you can see whether I really understand.
Anyway, thank you.
Anyway, thank you.
(08-07-2010, 04:58 PM)PJW Wrote:(08-07-2010, 09:11 AM)greensky52 Wrote: Referring to item 1, I am not sure I have got your point. In short, do you mean it should be decide by the actual situation of shunt route?
If the shunt route is for a pemissive movement, it is no need to consider the overlap. Such as shunting a train to couple with another one which stop in the platform?
If it is an unpermissive movement, overlap have to be considered for safety. Such as wrong direction movement?
In short Yes, there is no one answer for shunt routes-
a) sometimes they prove all tracks clear and have overlaps (just as if main routes),
b) sometimes they prove all tracks clear in route itself but not have any overlaps at all,
c) sometimes they prove all tracks clear in the route itself but not in the overlap yet still lock an overlap
d) sometimes they prove only some tracks clear in the route itself and would therefore have no overlap at all.
In general signals are approach locked when the signal clears.
For a main signal, occasionally it is worth providing "comprehensive approach locking" such that the locking is only actually imposed (or can be immediately released) if all the tracks are clear back to the sighting point of the furthest signal which changes aspect when signal concerned is replaced to danger. However it can be expensive to provide and takes time to test, so only provide where particularly useful.
Conversely a GPL signal is only visible for a short distance and the maximum speed of train approaching a GPL is only 15mph. Therefore comprehensive A/L is "always" provided (since it is cheap and simple).