Yes having now had a chance to look at it, the directional treadles just beyond G38 are to initiate the crossing if the outer strike-in had been inhibited by the signal being At Red And Free of Approach locking.
If this signal was later able to be cleared, then the crossing would be initiated immediately but where this could lead to the train arriving at the road too soon, then the actual aspect displayed to the driver would be delayed until this risk prevented.
However if this signal could not be cleared and therefore the driver needs to be verbally authorised to pass at danger (or in the event of a SPAD) then the crossing should immediately display alternate flashing red lights and give audible warning; the treadles at the signal are one of the diverse means of initiating this.
I also notice when I produced this layout that I positioned the strike-in for a 30sec strike-in rather than the 40sec which is needed for a double line crossing in also to give the 10sec "Another train Coming" feature. I did though draw attention to this error by placing a note to that effect against the opposition line's strike-in. I honestly cannot now remember whether this was a genuine mistake on my part, or actually deliberately included in order to show the sort of thing which happens in exam conditions and the way to get over it- in general there is not the time to rub lots out and attempt to correct, so making the examiner that you are aware and know how to rectify is often the better use of time and is far less prone to error and looking very confusing.
If this signal was later able to be cleared, then the crossing would be initiated immediately but where this could lead to the train arriving at the road too soon, then the actual aspect displayed to the driver would be delayed until this risk prevented.
However if this signal could not be cleared and therefore the driver needs to be verbally authorised to pass at danger (or in the event of a SPAD) then the crossing should immediately display alternate flashing red lights and give audible warning; the treadles at the signal are one of the diverse means of initiating this.
I also notice when I produced this layout that I positioned the strike-in for a 30sec strike-in rather than the 40sec which is needed for a double line crossing in also to give the 10sec "Another train Coming" feature. I did though draw attention to this error by placing a note to that effect against the opposition line's strike-in. I honestly cannot now remember whether this was a genuine mistake on my part, or actually deliberately included in order to show the sort of thing which happens in exam conditions and the way to get over it- in general there is not the time to rub lots out and attempt to correct, so making the examiner that you are aware and know how to rectify is often the better use of time and is far less prone to error and looking very confusing.
(20-09-2012, 04:22 PM)PJW Wrote: Haven't got access right now to my plan, so will respond later re detail.
However the treadles needed for an AHBC are:
1. For the outer strike in, which as you say could need to be inhibited if there is a signal within this area that then could require to have delayed clearance applied.
2. To detect a SPAD (to back-up the TC occupation) at such a signal
3. Treadles either side of the road crossing in order to determine directionality of the train and to raise the crossing afterwards
4. Strike-in for a "wrong direction" train (as per 1 but generally placed at lesser distance to reflect the lower speed for moves in the abnormal direction)
(20-09-2012, 12:56 PM)savitha kandasamy Wrote: Hi PJW,
with reference to your 2002 solved layout, I have one query. For upbranch, AHBC has a treadle placed near to SIP(40 sec).. My understanding is that, if a signal happens to be placed within the striking distance, the signal is approach released (delayed clearance control). But I am not able to understand the need for one more treadle near signal G38. Kindly clarify.
PJW