Why do you ask the question (a quiz question, or do you want to travel there?). I ask because the London train system is very complex.
For example does that 270 include Docklands Light Railway? I am not sure DLR is part of the Underground system.
And of course much of the "underground" system actually runs above ground. I used to live in North West London (Harrow, Wembley etc) and all the trains there run overground.
And of course there are many many "overground" stations in London that are not part of the underground system at all.
Note that Wiki link posted above does give details of some planned Underground extensions.
http://en.wikipedia.o...ements_and_expansions