(16-09-2010, 04:32 PM)greensky52 Wrote: 1 146C(M) routes to Down Main, but it requires 210A R, so is it still the opposing signal of 107? I feel I do not know the exact definition of opposing signal? It seems there is an overlap locking,is it overlap locking between 146C(M) and 107?
Yes because 107 can have an overlap over 210 reverse as denoted by the overlap symbol which includes BD and 211B but excludes BE. 107 authoorises a train to proceed up to 127 whereas 146 authorises a train as far as 118 so it is definite that only one of the two routes can be set at the same time. Note that there is no overlap beyond 118 over 205N so in that direction the point locking is sufficient, 107 locking 205N whereas 146 requires 205R. The same is NOT true in the opposite direction, due to the fact that there is an overlap beyond 127 with either lie of 210.
So you are right that you have not yet correctly understood opposing route locking. Look at this thread for discussion of opposing signals and also this one for a short presentation that may be useful.
Also my write up on Control Tables can be found here and there is quite a bit on opposing locking there
Quote:2 I remember in indirect route locking, if signal-A is parallel with another signal-B, and they have the same exit, then if B is direct opposing signal of signal C, A is indirect opposing of C. So, I think in 146A(M), 131B(M),131B©,127B(M) need to be route locking. Right or wrong?
You are correct that having decided that signal A is directly opposing signal C then you need to CHECK WHETHER signal B is indirectly opposing. It will be IF the point locking is insufficient; it is NOT shown as indirectly opposing if the locking is achieved by point locking. In the cases you list routes from 127 or 131 hold 211 reverse until train clears BD at which time 146A(M) can be set safely. The point availability test of 211 normal or free to go is adequate and therefore the opposing is not shown (even if it is actually present in circuits or data) as it is actually superfluous (= additional to that which is truly needed as it only duplicates the more fundamental locking). You do need to get your head around this; see item 1.
Quote:3 In 128A(S), why does it need 107A(M) in route locking? I think it does not belong to any of these situation:overlap locking, direct opposing locking, indirect opposing locking....
128B(S) is obviously directly opposing to 107A(M) and hence shunt can only set if the Down train has been timed to stand in platform. Regarding 128A(S) it does depend upon how the interlocking as a whole is designed; there must be SOME locking preventing 107A(M) with 210R being set simultaneously with 128A(S) but there are DIFFERENT WAYS of achieving this aim.
Personally I'd set and lock 215N whether 210N or 210R and hence achieve that way via pseudo point-to-point rather than route locking.
Similarly I'd make 128A(S) set and lock 210 N (again pseudo point-to-point giving trapping) and therefore that would also prevent.
However unless the CTs achieve by point locking, then would have to achieve via route locking.
It is always a bit dangerous just to focus on one element of the locking shown on a set of CTs because it is INTERLOCKING and you do need to consider the overall picture as well as looking at the various pieces of the jigsaw by themselves.
In the Exam an additional complication is that candidate does not do a full set of CTs and therefore the examiner has sometimes to judge an attempt without knowing for sure how the CTs not requested would have been completed; they can only assume that would be consistent with those CTs that were done. Of course in the exam the candidate does have the option when doing a particular CT to state an assumption re what the relevant portions of another CT would have shown.
You are doing the right thing trying to work out what is right, what is wrong and why- however it does suggest to me that you are not fully grasping the "big picture" and seeing where it all fits together.
PJW