In message , Soniakostas
writes
Has anyone of you ever heard in some odl trains especially the metropolitan
ones that there is a noise after the accriage of a machine like a generator
that periodically starts and stops, and it lasts for a few seconds.

Is it some kind fan ?? i noticed this starts when a train starts has been
running for a while and it isnt met in new trains (so loudly at least)

cheers !!
Sounds like the compressors to me.
--
Clive.