Many public transit systems now use RFID cards for fare payments. Sometimes, multiple cities purchase systems from the same vendor, or use the same design, such that if you put cards for both cities on the reader, it would try to look up an account associated with the other city's card number, and either not find an account (& thus report the card is invalid) or in a rare case, charge some other random person's account.

For the purpose of this question, payment card/ID systems "interfere" if an attempt to read one credential on another system produces an error signal or unexpected behavior, different from what would happen if someone attempted to use blank paper or cardboard as a credential. For good answers, they do not have to both be transit systems (e.g. if some credit card system or large university access ID card or hotel chain key cards interfered, that would be good to know).

As a result of knowing the answer, a traveler with more than one city from a given interference set on his/her itinerary would know to keep the cards separate from one another and only hold up the correct city's card to the reader, instead of e.g. holding up a whole wallet or purse that contains the intended card as well as interfering ones.

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    The number of combinations of valid answers here is undoubtedly very large. And it's not just transit cards either, credit cards with RFID disrupt transit systems in certain cities. I'm afraid I'm going to vote to close as "too broad". – Greg Hewgill May 20 '16 at 2:39
  • > credit cards with RFID disrupt transit systems in certain cities. - Confirmed in Vancouver. I have a beautiful Decadent Minimalist wallet and I can only use the Compass card if I slide it out, otherwise my RFID Visa card on top interferes. – chx May 20 '16 at 2:43
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    The number of potential combinations (esp. binary combinations) is undoubtedly very large, but I suspect there are a relatively small number of sets which actually interfere with each other (i.e. anything within the set interferes with anything else in the same set). This seems more specifically answerable than many much broader questions on this site. – WBT May 20 '16 at 2:43
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    The number of potential combinations that would have to be tested to determine which combinations were indeed disruptive would be astronomical. Best you could hope for right now would be anecdotal information and even that might not drill down to which cards in the wallet were actually doing it since people frequently have multiple chip cards. – user13044 May 20 '16 at 3:37
  • @Tom These things are already being tested in practice and interferences are being discovered, even without requiring people to go out and test anything specifically for this question. As to which one in a wallet specifically causes interference, those experiencing the interference have to figure that out anyway in order to reliably use their transit tickets (e.g. "If I stop carrying that one in the same bundle, this one works."). – WBT May 20 '16 at 13:48

Pretty much every card follows the standard ISO/IEC 15693 or ISO/IEC 14443 which means they operate on 13.56 MHz and will potentially interfere with each other. You might be lucky here or there but in general any two contactless cards will cause interference.

  • Frequency isn't the only factor; data can be encoded differently and that can affect whether or not a system recognizes a card as being part of the same system. Potential interference is more broadly possible than the actual interference that really happens: lots of folks put short stacks of non-interfering contactless cards up to readers every day without having errors reported or observing unexpected behavior. – WBT May 20 '16 at 13:54
  • @WBT High-level encoding would be relevant if there are any interference mitigation build in (e.g. Wifi does that) which those standards haven't – that simply wasn't a property considered when designing them. Low-level activation and encoding is similar (that's the point of those standards) and the reader usually can't make out a single card let alone choose between multiple. – neo May 20 '16 at 14:16
  • As said, in practice you might have luck with a particular combination of reader and cards due to the specific (unintentional) properties but if you change any of those that might change. A stack of card might work at one ticket gate and not at another, etc. Apart from testing your particular combination there can't be anything said in general. – neo May 20 '16 at 14:16
  • "The reader usually can't make out a single card let alone choose between multiple." I disagree. Lots of readers successfully do that every day. It's also fairly easy for a traveler who spends more than a little time in any particular city to detect a pattern of consistent interference if and only if both cards are presented together. – WBT May 20 '16 at 14:22
  • @WBT That's what I have said? Sometimes it works, sometimes it doesn't but that's not a property that can be discovered without actually testing it in the wild. And that's the reason this question is closed here. – neo May 20 '16 at 14:25

The city of Boston, MA, US uses a system called CharlieCard, and the city of Pittsburgh, PA, US calls their system ConnectCard, but both apparently work on the same standards and will interfere with each other if carried or used together.

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