And the first dividends of Transport For London's opening of its data to the public have started flowing in; a chap named Matthew Somerville has created
a real-time map of trains on the London Underground, displayed on a Google Map. The source code is available
here. Somerville also has
a similar map for National Rail trains, though, due to the more limited data published by National Rail, it can only show trains due to arrive at one of several stations. (Let's hope that National Rail see the benefits of opening their data soon.)