I did see something in the paper a while ago about a proposal to construct one, actually, but I'll believe it when I see it. The Virgin express intercities are now down to less than five hours, though, which isn't too bad.

But seriously, £15bn (the price quoted on the Today programme this morning) seems a little daft for Yet Another railway line through London. At the low end of the estimates, you could build a space elevator (http://en.wikipedia.org/wiki/Space_elevator_economics) for that.