If the OP has a US credit card, would the machines accept it? (Prompted by
the fact that most Paris Metro machines don't accept even UK cards.)

Just returned from a week in London. My US MasterCard worked everywhere,
including Tfl. Occasionally a Tesco clerk would insist I provide a pin
number, but when I explained it was a credit transaction, not debit, they
went ahead and successfully completed the transaction.

Rich