Is it one of level crossings by the West London Line at Willesden? I
vaguely remember there's one on the West London line itself just after it
leaves the WCML, and another in the Eurostar depot at North Pole. However,
as to whether these are nearer than Barnes and actually inside zone 2 I
don't know. I'm probably imagining their presence anyway.

Angus

Well remembered, Angus - that's the exact one I had in mind: I only
"discovered" it myself recently when en route from Clapham Junction to Watford
on the W.L.L.

This is within the Zone 2 bondary as far as I can make out, since both Olympia
and Willesden, the stations either side, are.

Marc.