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What would happen if I held my wallet, containing two credit cards, to the RFID reader on the London Underground turnstiles?

If I had one credit card in it it'd, I assume, debit the ride from that one credit card, but if I had two, back to back, in my wallet, would it try to debit both of them?

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    Buy an RFID blocking wallet and either take the card out or use contactless payments on your phone. It's much safer.
    – SnakeDoc
    Commented Nov 11, 2022 at 18:05
  • Not saying that the underground isn't relatively expensive, but perhaps try one time and see which card(s) get charged?
    – BruceWayne
    Commented Nov 13, 2022 at 1:03
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    Speaking as a software developer, the most likely scenario is that the code was written under the express expectation of a single card being read, and simply will not handle having more than one card's information. This means that either the reading will immediately fail (this is my guess, that the hardware will reject the reading before the software is even involved), or the software will crash, or (best case) the software just takes the first reading and runs with it.
    – Flater
    Commented Nov 13, 2022 at 23:17
  • Related question from Ask Different - apple.stackexchange.com/questions/364021/…
    – Tetsujin
    Commented Nov 14, 2022 at 9:40
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    @Flater that seems likely, another likely result is that the cards are not truly read at the "same time" as they will be physically separated by a small distance, so one will be read a microsecond or two before the other. The first one should succeed and lock out further inputs for a second or so
    – Darren H
    Commented Nov 14, 2022 at 11:18

4 Answers 4

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I would think “it depends”, and it’s difficult to guarantee you’ll always get the same behaviour.

ISO 14443 smart cards, and that includes EMV contactless payment cards as well as MIFARE cards (including Oyster cards) have an “anti collision” system that enables a reader to detect multiple cards (and actually, at least in theory, to identify each of them and talk to each of them independently).

The EMV contactless specifications state that if the reader detects multiple cards, it should not attempt to process the payment.

However, the big issue is that the performance of cards may be very different (especially related to the size and shape of the antenna embedded in the card), so in certain conditions (distance from the reader, obstacles, orientation, order of the cards…), one card may be detected but not the other, especially in edge cases (when the cards are still a few cm away from the reader).

There are so many factors at play that this may not be consistent, so for the same combination of cards, you may have either one card detected, the other card detected, both detected, or none at all.

If two or more cards are detected, no transaction should take place (which means you are now annoying people behind you as you take time to take one of the cards out of your wallet to place it on the reader). If only one card is detected, you may end up in situations where one card is detected when you touch in and another when you touch out. Not sure what happens in this case, but I wouldn't be surprised if you could end up with two "incomplete" trips charged at the maximum fare!

Your best bet, by far, is to make sure you only ever present a single contactless card.

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    In similar EMV-based checkin/checkout systems without turnstiles (in the EU), there's a serious risk that holding a whole wallet to the scanner is going to check you in on one card and then check you in on another card when you actually want to check out, earning you not one but two maximum fares. I guess the turnstiles might not let you check out with a card that's not checked in (because there's a separate reader for checkin and checkout). But you probably can run into the same issue with the pink (transfer) readers.
    – TooTea
    Commented Nov 11, 2022 at 13:13
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    The TFL contactless system does not reconcile touch in and touch out in real time, that happens in the back office later. Even on oyster which does track "in-travel" state in real time I think they prefer to slap people with maximum fares than trap people in the station. So touching in with one card and out with another will get you two "incomplete journeys" which can get expensive.
    – Peter Green
    Commented Nov 11, 2022 at 13:18
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    In Singapore, a warning is frequently shown in MRT stations and buses that both card will be charged with maximum fare, So take it out and tap. (We have entry and exit controls on bus and trains, unlike UK).
    – Anish Sheela
    Commented Nov 11, 2022 at 14:14
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    @TooTea Shameless plug for the Berlin system: there are no gates anywhere, only random checks on paper tickets (or single-purpose plastic cards for contract subscriptions). Perhaps counter-intuitively, avoiding technology made it fairly simple and foolproof! and by selling the tickets in optional multi-packs, irregular travelers won't have to buy them frequently and regular travelers probably buy concession tickets anyway
    – Stack Exchange Supports Israel
    Commented Nov 11, 2022 at 18:38
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    @Mindwin Because if the passenger never checked in, the transit system would lose some amount of income otherwise. Same as charging maximum fare if you check in but somehow skip checking out/ In either case they have no idea how far you rode, so they can’t charge based on how far you rode, and they default to just charging maximum fare (which is usually the default behavior when encountering any erroneous transaction in such systems).
    – Austin Hemmelgarn
    Commented Nov 11, 2022 at 20:04
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The official TFL page calls this "card clash"

https://tfl.gov.uk/fares/how-to-pay-and-where-to-buy-tickets-and-oyster/pay-as-you-go/card-clash

Specifically they list these problems

If you don't, the yellow card reader may:

  • Not read any card and the ticket gate won't open
  • Not take payment for your journey (at a freestanding yellow card reader) so you could be charged a penalty fare
  • Read one card when you touch in and another card when you touch out, charging two maximum fares for your journey
  • Take payment from the wrong card
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  • Excellent information.
    – Fattie
    Commented Nov 14, 2022 at 12:47
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If you have two contactless cards in your purse or wallet and hold it over the reader, the payment will normally be rejected.

However, to be safe you should remove the card from your purse or wallet before using it - as you would for any other payment.

Source: https://supportcentre.natwestinternational.com/Searchable/913246372/If-I-have-two-contactless-cards-can-I-be-charged-twice.htm

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    I'm pretty sure whoever wrote the page you have linked to did not have touch-in/touch-out transit systems in mind.
    – Peter Green
    Commented Nov 11, 2022 at 13:28
  • Once card can be shielded inside metallic foil. I bought some cheap card holder foil bags and keep my cards in such. And pay with phone so google konws even more about me ¯_ (ツ)_/¯
    – akostadinov
    Commented Nov 14, 2022 at 11:46
  • "If you have two contactless cards in your purse or wallet and hold it over the reader, the payment will normally be rejected." Unfortunately that sentence is wrong. In a perfect laboratory setting it will work (both will be recognized, and hence both rejected) but in the real world unfortunately it's commonplace that only one is recognized.
    – Fattie
    Commented Nov 14, 2022 at 12:49
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If the reader detects two cards, it should throw an error.

If it only detects one - and that is quite possible, think how many times you need to take a couple of tries at touching your card - it will charge that one. Just hope it only detects the SAME one when you touch out!

Mistakes, as long as they don't look like attempts at fraud, are cheerfully corrected. But YOU need to check your statement and make a claim.

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