The right answer (in practice, though I admit the question is fun) is to "use your debit/credit card" (on the Tube, for instance) for the train or "set up auto-

The right answer (in practice, though I admit the question is fun) is to "use your debit/credit card" (on the Tube, for instance) for the train or "set up auto-load if your transit system doesn't accept credit/debit cards" (on BART, for instance).