About the steam whistle sound that people report hearing from the London Underground - and this might not exactly be news to the many London based commuters and frequent tube riders.
Although every one knows that the Underground trains are electric, a lesser know fact is that each train still has it's own whistle. I was watching a recent documentary about the tube where, in all honestly, I heard that whistle more often than I have in 4 years travelling on the tube.
So I surmise that when a 'ghostly' whistle is heard in the night, it could be one of the trains heading back to the depot letting some of the track workers know of it's approach. Could also be one of the repair and maintenance trains too...