But the point isn't "How do the prove they've paid", the point is "Do they have a valid ticket?" and there's a world of difference as I pointed out with the Annual Gold Card discount on other ticket purchases. I get 1/3 off a ticket to Manchester if I buy it whilst presenting my Annual Gold Card London commuter card. I can shout and scream and cry until I'm blue in the face, produce banks statements, receipts, little pieces of paper to show that I've paid my fare to Manchester, but if I am not carrying my annual season ticket Gold Card then the ticket isn't valid - further, my Gold Card isn't valid unless THAT is accompanied by the same numbered photo ID card. So I need THREE things before my ticket is valid. OK, I've paid the fare, but that's not enough to make it a valid ticket.