Thanks David, Bob, FJ and Stephen.
Interesting that a warning was broadcast before use of the whistle. We have a similar situation in the rail industry where the use of detonators is almost extinct in case it scares the fairies. No matter that their use may well save someone's life.
I have always been under the IMPRESSION that a whistle blown with steam will give a different sound than the same whistle blown with air but have no factual basis for that impression.
These days one is unlikely to be able put that impression to the test.
We did put a steam loco whistle on a diesel loco 50 odd years ago but by the very nature of the steam unit being a 'whistle' rather than a 'horn' it pretty much drained the loco's main reservoir air in an instant. (This, of course, should have been obvious from the start.)