Spyke wrote in message ...

To change the subject slightly, there is a similar "singing" sound from
the overhead power lines at Ealing Broadway station; at least, that's
where it appears to be coming from. It is constant, i.e. it doesn't
start up just when a train is approaching, and it is quite loud. I
haven't noticed it on other lines with overhead electrification, so I
wonder what is causing it?

I was there the other day, and wondering what the strange noise was. It
seemed to me to be more of a beeping than a singing though.

The singing noise comes from a box in the six foot on the main lines
that looks like a cable termination box - I suspect it's a Tuning Unit
for TI21 jointless track circuits, having seen a similar one in the
National Railway Supplies catalogue. The purpose is to cause the
circuit to resonate at the defined operating frequency which is in the
audio range, and they are installed at track circuit boundaries.