"Keith Marshall" wrote in message

I have not heard this anywhere else on the Underground except at
Turnham Green

Listen carefully and you'll hear it at just about any over-ground tube
station where there is a log stretch of unbroken rail (ie. no
points/crossovers). I've heard it on just about every Piccadilly,
District and Central line open-air station I've ever used. And IME
the faster the train is going the more pronounced the noise. Clearly
what is happening is that the vibration of the train is setting up a
resonance in the rail, which will travel some way ahead of (and
behind) the train. The sound is there, but IME less pronounced, at
sub-surface stations -- probably because the rails are held more
tightly on the concrete sleepers/floors so the resonance is better
damped compared with over-ground sleepers and ballast. Also listen
carefully and I think you'll find the sound is also Doppler shifted.

Presumably the sound seems louder because the Piccadilly Line trains are
normally non-stopping and running through the station relatively fast. There
aren't many other stations with Tube trains passing through at speed.

I also wondered if the steel girder bridge immediately to the west of the
station plays any part in the noise?