"Dave Arquati" wrote in message
...

The Ruislips are 3-tracked already, mostly for freight purposes I think.

Not really. At South Ruislip traffic from Marylebone can *only* take the
platform road, whilst traffic from Greenford can only take the centre road
(hence the reason that, when Paddington diversions are on, no trains stop at
South Ruislip). So, effectively, the outer two tracks are the up and down
Marylebone, whilst the easternmost pair are the up and down Greenford (the
up line gives access to both routes). At West Ruislip the centre road is an
up through line (although, IIRC, it is bi-directionally signalled - perhaps
Roger can correct me on that).