Sometimes the London Underground manage to find drivers willing to do this. It happened today... I was travelling between Green Park and Holborn.

Seriously though... if you're into "train terrorism", I think attacking underground systems is more effective - especially a packed system like London, and of course its been done before.

My guess is that in a "spread out" country like the US, it would always be more cost effective to invest more in airport security infrastructure than upgrade/install new train systems. (Much as I like trains.) The "whole environment" thing might push the balance more towards trains I suppose.