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.

*Keith*
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