Thomas Crame wrote:

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.

Thank you for the explanation.

--
John Ray, London UK.

Mail to mefp49 is unlikely to be read. I can be contacted at xcf70 (same
ISP).