Thus electronic ticket systems :) The local government her in Oslo, Norway are working on something similar[0]. It should have been finished in 2005.
Now after massive cost overruns they have scoped it down and will soon demolish part of the system (the ticket barriers at the railway station), but still keeping some of it. And of course they have now also started to talk about a new system, this time requiring everyone to have a smartphone with them ( what will happen if your battery runs flat?).