I was joyriding on the North London Line and happened to read the message that scrolls forever on the LED display.
WELCOME TO THE LONDON OVERGROUND SERVICE TO STRATFORD THE NEXT STATION...
Now quite aside from the redundancy of London (although I like to imagine somebody jumping up and shouting 'This isn't Tokyo?') the word overground is curious. It is what is known these days as a retronym. There was a time when all trains were overground unless otherwise specified, but London is now so filled with metal worms sliding gracelessly beneath the city, that they need to tell you that this train is overground.
It's the same principle by which we have a landline, organic food, or live music.
Mind you, as the train has windows, you wouldn't think that the word was utterly necessary.
P.S. A philosopher friend of mine once pointed out how the use of the word music has changed: that you can now ask 'What music do you have?' and someone can reply 'I have Caruso singing Poker Face.' The idea would be incomprehensible a couple of hundred years ago.