"Jack Taylor" wrote in message
. ..

"Clive Coleman" wrote in message
...
Sounds like the compressors to me.

Almost certainly - but you'll only hear them if you're travelling in the
second or third cars of each four-car train. With the lack of motor noise
from those trailers (power is provided from the driving cars at each end)
the sound of the compressors is very noticeable.


Compressors are only mounted on the trailer (non-motor) cars.

They supply air for the braks (as mentioned), the doors and for the
automatic acceleration equipment on trains prior to electronic control.