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?

  • 1
    I'm sorry, but I don't really think this question is on topic for this site. (The answer is each card has an id number, they are just all getting logged centrally)
    – hardillb
    Jan 14, 2020 at 13:13
  • @hardillb that's fine. I struggled to find which site it belonged on - it's not a programming problem. So I thought it seemed more IoT related than anything else. Do you have a link to any articles or more information about how it works? Or alternatively, can you recommend which stack to ask the question on? Jan 14, 2020 at 15:35

1 Answer 1


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.

Not the answer you're looking for? Browse other questions tagged or ask your own question.