The real fun goes back to the days of dear old "BRITISH RAILWAYS" Yes, they dumped it on the track. The trick was remembering not to flush while the train was in a tunnel.
Nine times out of ten you would get away with it, on the 10th there would be a train approaching from the other end of the tunnel, and as the valve opened, the air presure would rise and whoosh you would get your own back.

Gareth (Nostalic for Steam and the old Mark 1 carriges.)