We get the same effect at Hassocks on the Brighton line. It is
vibration of the conductor rail.
But at Turnham Green it starts well before the outside conductor rail
switches position.

Certainly IME this isn't necessarily the conductor rail(s). I've
noticed it many times on BR trains with no conductor rails. I am also
doubtful if the girder bridge near Turnham Green has a significant
effect; again I've heard this many times on bridgeless stretches of
track. The most that bridge will be doing is acting as a sound box
(like the body of a guitar).

Keith
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