Anti-preselection......

[onestrangeday]

Hi PJW:

I have included the track layout for better understanding.
I agree with you, maybe the risk is not high, but it can be improved and be better designed. (has this ever happened in UK)


once again thanks for your explanation

The clause in the EFTR is certainly clear to me and it does appear from your description that the 2nd route request was stored, either deliberately or accidentally, by the system as it should not.

If I understood your account correctly, there was no incident involving train 1 which held its route until clear of the points. The second route that had been stored then started to set and called those points but then immediately cancelled itself, fundamentally because train 1 had fulfilled the conditions which it was also looking for (I'd need to see a layout to understand this better). Therefore undesirable certainly but not actually directly dangerous; however it does reveal that the interlocking not performing as specified which in itself does raise concerns of course.

Also I am a bit "interested" why the route release conditions for route 2 could have ben satisfied by a train on route 1; it is certainly possible that the same crucial track sections involved in each case, but once the release had been effective for route 1 then it should have immediately destroyed the memory of the satisfaction of sequence rather than leaving them (perhaps it was just a short duration timing thing that would not be significant if the primary failure had not occurred) primed to do the same for another occasion / route.

So:

1. Your anti preselection clause seems fine in that it should have prevented what you reported happened.

2. I do think that the interlocking functionality does need to be investigated. Would seem that the implementation is defective in some way, possibly due to a site fault but I suspect far more likely to be a design error and yes it could well be "in concept" and thus generic rather than a one off in that particular circuit (assuming that this is route relay interlocking but analogous in the case of computer based interlocking)

hi PJW:

thanks for your detailed explanation. one of the reason I ask about this is that there was actually a points failure due to the lack of anti-preselection function included in the interlocking.

The incident happened in depot (signallers have to set route by themselves not through ARS as in the mainline) where the controller set a route (for example Signal 1 to track circuit 1) and along the route there is a point A which has to be in normal position for the designated route. After setting the route and point A was already locked in normal position and train driver received the permission from signaller and started to move to destination track circuit 1.

But for some reason while the train was proceeding to track circuit 1, the signaller pre-set another route (for example signal 2 to track circuit 2) and along the route there is the same point A which has to be in REVERSE position for the route, but since the previous route already taken by signal 1; therefore point A had to be in normal position before being released by sectional locking. Consequently, the route being stored (signal 2 to track circuit 2) and kept the signal 2 at 'red' aspect.

Here comes the rare case.
As the train proceeded to destination track circuit 1 and passed the track circuit A (where point A are), the points were released since the train had proceeded to the next track circuit and cleared the track circuit A. So the points A started to move to the REVERSE position according to the previous stored route (signal 2 to track circuit 2), but at the same time the sequence of occupation of the previous train also fulfilled the condition of automatic route restore for (signal 2 to track circuit 2). So as the points started to move to REVERSE position the route was canceled automatically by the system, therefore the controller received a points failure alarm.

until now, we have informed the depot signaller not to pre-set any route before the previous train arrived its destination track circuit, just to make sure the same thing won't happen again in the future.

So I am not sure whether this could be an outstanding issue that could raised for the extension line project for the contractor to rectify the system accordingly. (I am also not sure whether this is the 'limit' of signalling system, that is not possible to exclude something like this)

I have also tried to find the EFTR (employment functional and technical requirement) document 9 yrs ago, which have stated the function of anti-preselection as following shown:

"
5.2.5 Anti-preselection
Any calls to set routes or move turnouts shall only be effective if such routes or turnouts are available and free to move at the time the call is made. It shall be necessary to remove such calls and re-apply them when the routes or turnouts are available and free to move.
Calls applied to set routes or move turnouts which are not available or free to move shall have no effect whatsoever. "

thanks