"John Salmon" wrote in message
...

"Jack Taylor" wrote
At West Ruislip the centre road is an
up through line (although, IIRC, it is bi-directionally signalled ...

Not according to Quail.

If a stopping train is let out of Marylebone in front of a fast, the first
opportunity for the fast to overtake is Princes Risborough. In the up
direction there are overtaking opportunities at High Wycombe (where the down
platform line is reversibly signalled), and at West Ruislip (using the Up
Platfrom Loop).

Peter