Danny252 wrote:I'm intrigued by the description of the crossing's operation from the RAIB brief. Should this be taken to mean that AHBs are not provided with a track circuit (or axle counter section) over the crossing itself, which forces the operation of the crossing when occupied, regardless of the normal approach sequence?
The IRSE's
Railway Control Systems published in 1991 gives typical circuits for AHBs (and other types of crossing then current) - fig 6.5 & 6.6 & 6.11 & 6.12 for double line bi-directions strike ins. These do show short track circuits for the crossings, but a note says these are only provided for specific risks - notably agricultural vehicles occupying and short circuiting the track circuit causing the crossing to fail. Where they are not provided the strike in TC extends to include the crossing.
Fig 6.10 also shows a separate short TC for the crossing for single lines, but of course for trains running the other way the strike-in TC becomes the strike-out and vice versa. Single track may in part explain why middle tracks are more common in Scotland.
I don't know if this logic is still valid as most track laying vehicles I have seen lately seem to have rubber tracks rather than the chain-link tank type tracks which seemed more common when I was young. However, there may well still be places where this is an issue.