In London, and soon in Paris also, contactless bank cards can be used to pay for public transport.
The system tracks a user's journey, through the "tapping in" and "tapping out" of the user; and then charges them the appropriate amount for the journey they have taken.
My question is, how is this information tracked? I assume the system can't be writing information to my bank card, so is it sending the data to a central system somewhere? Or does it work another way?