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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?
There are a few references to this on the Wikipedia page for the Oyster card:
Since the Oyster readers cannot write to a contactless card, the reader when touching out is unable to display the fare charged for the journey, as the card does not have the starting point stored in it. This is calculated overnight once the touch in and touch out information is downloaded from the gates and collated.
(emphasis mine)
There are a few more details in that section. Sadly the source in that section does not point anywhere.
