233 points
These are switch diamonds which have been separately numbered (as per latest NR practice but also see below) from the associated single end 231. Alex hasn't stated the practice he followed but a reasonable assumption from this CT is current Network Rail with SSI.
Dead track locking:
It is obviously correct that CJ locks both ways. It is also correct that 233 should be prevented from normalisation if the closest portion of BJ were occupied since this is FOUL; therefore the (BJ or 229N, 231N) is correct.
However I would not have put the (CK or 232N) entry; if there is a train leaving the Up Branch this would prevent the signaller from setting for example 129B(M) had 233 been left reverse after a previous movement to the Down Branch. Foul dead track locking is primarily provided to stop the signaller from moving points induividually into a position which defines a route over which they might authorise a handsignalled movement that APPEARS to be OK (all tracks in line of route clear) yet when there might be a train so close to that route that there is insufficient clearance. In this case normalising 233 gets them to point AT the occupied track CK and indeed 232 are being locked the wrong way for a move from 144 (for example).
Now considering the N to R locking. I don't think it is strictly necessary to prove BJ clear; however to do so is not restrictive. Hence it is probably a good idea to do what Alex has done and include it; after all it would have had to lock the points had 231 been numbered as another end of 233 (obviously such a numbering scheme would only have been appropriate on a simpler track layout without the slip connection 229). The locking that has got MISSED is what should be preventing 233 from being reversed if there is a train on the Up Main which has just passed over them in the normal lie but has travelled an insufficient distance to be clear i.e. there should be an entry (CK or 232R) in the top cell of the second column of the CT.
Set By:
I agree with the entries in the lower cell of the third column for those routes that call 233 normal. Alex has obviously followed recent practice by giving shunt routes overlaps since 123A(S) is included [the REAL one not the examiners' mistake!]. There is a hint that this is the practice tacitly assumed by the layout (but remember candidates can CHOOSE their practice) as the SE/SF joint is drawn as an overlap and labelled "shunt 60m". There is no need (and better not to) annotate 146A(S) with a note "requires 124 set"- this is not relevant to the points CT. Indeed the route box requirement is actually "requires 124 off" (i.e. aspect cleared and therefore approach locked, rather than just route having been selected)- subtly different.
Conversely I don't agree all the routes calling the points Reverse; 123C(S), 136A(M), 182B(M). I can see why the routes over 232R would be "happier" to have 233 set for a parallel move, but as explained above re the dead track locking, the Down Main moves "want" 233N when 231N. Assuming that we need to be able to run a train on the Down Main simultaneously with one from the Up Branch (of course!), then "something has to give". Hence the Down Main "wins" and the aspects of routes along the Up Branch must be content to prove CJ clear [#1]. Note that a train from 123B(S) would continue to lock 232N whilst on CJ and thus routes such as 136A(M) would only be able to set once CJ clear.
[#1 The condition on CJ to prevent aspect replacement for a parallel move should be expressed as a track sequence- the position of switch diamonds can't be relied upon to give flank protection and if an invalid movement is made the prove may either carry on diagonally or more likely derail].
Routes 138B(M/C) were overlooked!
Route holding
Column 4 should be amended as per column 3 comments.
The associated columns 5-7 are generally Ok and bracketing the routes together that have the same tracks is a useful exam technique to save valuable seconds.
Track BF is missed from 123A(S)- easy done when the track name is on a portion of the section which is not traversed BUT you need to train yourself not to make such errors- be conscious of the IBJs and go look for the name in each case! Also te track names have simply been put in one list; there is an overlap portion that should time out in the same way as it does for 113A(M)(W). Actually there is no need for the route locking to include the overap tracks BJ, CJ as these are dead locking tracks; whereas tracks within the ROUTE have 15 second anti-track-bob protection and thus the route locking over dea tracks should be shown, there is no such additional locking once the overlap is entered- doesn't do any harm but adds nothing so shouldn't record. BK is certainly wrong- don't just put all the overlap tracks down without thinking; train is beyond the points by the time only BK occupied.
For the locking in the opposite direction, somewhat similar comments re 182C(M). In this case we DO need to list EA as ths is part of the overlap PRIOR to the commencement of the deadlocking of the points. This overlap shoul also time out on EB for a suitable time but the brackets should be arranged that EA is outside the bracketted condition so that the locking is held indefinitely if this becomes occupied following a SPAD.
Overall, not too bad- the switch diamonds made the foul TC locking a bit tricky and you got much of it.
A useful tip is to compare the tracks listed in each of the maintained locking entries (col 5) with those included in the dead locking (col 2)- route locking should go up to AND INCLUDE, overlap locking should go up to BUT EXCLUDE. If there is a foul track and the relevant route passes over it, then yes INCLUDE that track within the list of route locking tracks. Conversely don't put tracks in this list that are after the relevant train has passed beyond the dead locking tracks.
Remember that the locking on points is imposed when they are first called by a route (col 3) and then held until that signal is free of approach locking (col 4). They also need to be held by a train that has passed beyond that signal but has yet to pass safely beyond the points (i.e. to be totally clear of them- this means not on a foul track)
[BEWARE those familar with RRI rather than SSI.
RRI would correctly have slightly different route locking entries etc. since it is usual practice for there to be no anti-track-bob protection and thus route locking would not generally show any tracks which dead lock]
Need also to get a bit more familiarity re overlap locking; certainly don't forget to make them time out!
Comments on the other CTs will have to wait until later occasion!
These are switch diamonds which have been separately numbered (as per latest NR practice but also see below) from the associated single end 231. Alex hasn't stated the practice he followed but a reasonable assumption from this CT is current Network Rail with SSI.
Dead track locking:
It is obviously correct that CJ locks both ways. It is also correct that 233 should be prevented from normalisation if the closest portion of BJ were occupied since this is FOUL; therefore the (BJ or 229N, 231N) is correct.
However I would not have put the (CK or 232N) entry; if there is a train leaving the Up Branch this would prevent the signaller from setting for example 129B(M) had 233 been left reverse after a previous movement to the Down Branch. Foul dead track locking is primarily provided to stop the signaller from moving points induividually into a position which defines a route over which they might authorise a handsignalled movement that APPEARS to be OK (all tracks in line of route clear) yet when there might be a train so close to that route that there is insufficient clearance. In this case normalising 233 gets them to point AT the occupied track CK and indeed 232 are being locked the wrong way for a move from 144 (for example).
Now considering the N to R locking. I don't think it is strictly necessary to prove BJ clear; however to do so is not restrictive. Hence it is probably a good idea to do what Alex has done and include it; after all it would have had to lock the points had 231 been numbered as another end of 233 (obviously such a numbering scheme would only have been appropriate on a simpler track layout without the slip connection 229). The locking that has got MISSED is what should be preventing 233 from being reversed if there is a train on the Up Main which has just passed over them in the normal lie but has travelled an insufficient distance to be clear i.e. there should be an entry (CK or 232R) in the top cell of the second column of the CT.
Set By:
I agree with the entries in the lower cell of the third column for those routes that call 233 normal. Alex has obviously followed recent practice by giving shunt routes overlaps since 123A(S) is included [the REAL one not the examiners' mistake!]. There is a hint that this is the practice tacitly assumed by the layout (but remember candidates can CHOOSE their practice) as the SE/SF joint is drawn as an overlap and labelled "shunt 60m". There is no need (and better not to) annotate 146A(S) with a note "requires 124 set"- this is not relevant to the points CT. Indeed the route box requirement is actually "requires 124 off" (i.e. aspect cleared and therefore approach locked, rather than just route having been selected)- subtly different.
Conversely I don't agree all the routes calling the points Reverse; 123C(S), 136A(M), 182B(M). I can see why the routes over 232R would be "happier" to have 233 set for a parallel move, but as explained above re the dead track locking, the Down Main moves "want" 233N when 231N. Assuming that we need to be able to run a train on the Down Main simultaneously with one from the Up Branch (of course!), then "something has to give". Hence the Down Main "wins" and the aspects of routes along the Up Branch must be content to prove CJ clear [#1]. Note that a train from 123B(S) would continue to lock 232N whilst on CJ and thus routes such as 136A(M) would only be able to set once CJ clear.
[#1 The condition on CJ to prevent aspect replacement for a parallel move should be expressed as a track sequence- the position of switch diamonds can't be relied upon to give flank protection and if an invalid movement is made the prove may either carry on diagonally or more likely derail].
Routes 138B(M/C) were overlooked!
Route holding
Column 4 should be amended as per column 3 comments.
The associated columns 5-7 are generally Ok and bracketing the routes together that have the same tracks is a useful exam technique to save valuable seconds.
Track BF is missed from 123A(S)- easy done when the track name is on a portion of the section which is not traversed BUT you need to train yourself not to make such errors- be conscious of the IBJs and go look for the name in each case! Also te track names have simply been put in one list; there is an overlap portion that should time out in the same way as it does for 113A(M)(W). Actually there is no need for the route locking to include the overap tracks BJ, CJ as these are dead locking tracks; whereas tracks within the ROUTE have 15 second anti-track-bob protection and thus the route locking over dea tracks should be shown, there is no such additional locking once the overlap is entered- doesn't do any harm but adds nothing so shouldn't record. BK is certainly wrong- don't just put all the overlap tracks down without thinking; train is beyond the points by the time only BK occupied.
For the locking in the opposite direction, somewhat similar comments re 182C(M). In this case we DO need to list EA as ths is part of the overlap PRIOR to the commencement of the deadlocking of the points. This overlap shoul also time out on EB for a suitable time but the brackets should be arranged that EA is outside the bracketted condition so that the locking is held indefinitely if this becomes occupied following a SPAD.
Overall, not too bad- the switch diamonds made the foul TC locking a bit tricky and you got much of it.
A useful tip is to compare the tracks listed in each of the maintained locking entries (col 5) with those included in the dead locking (col 2)- route locking should go up to AND INCLUDE, overlap locking should go up to BUT EXCLUDE. If there is a foul track and the relevant route passes over it, then yes INCLUDE that track within the list of route locking tracks. Conversely don't put tracks in this list that are after the relevant train has passed beyond the dead locking tracks.
Remember that the locking on points is imposed when they are first called by a route (col 3) and then held until that signal is free of approach locking (col 4). They also need to be held by a train that has passed beyond that signal but has yet to pass safely beyond the points (i.e. to be totally clear of them- this means not on a foul track)
[BEWARE those familar with RRI rather than SSI.
RRI would correctly have slightly different route locking entries etc. since it is usual practice for there to be no anti-track-bob protection and thus route locking would not generally show any tracks which dead lock]
Need also to get a bit more familiarity re overlap locking; certainly don't forget to make them time out!
Comments on the other CTs will have to wait until later occasion!
PJW