OK, I know it's not allowed normally because you must have a railcard to back up the discount entitlement. But, what about two senior railcard holders from "up norf" who occasionally visit London but not at the same time. Is there anything to stop them sharing an Oyster card with discount set? AFAIK the only detail taken from the railcard is the expiry date, but is that just used to tell the Oyster system when to stop discounting, or do revenue staff with their MOVie readers see the date. If they do, would they be concerned if it didn't match? Obviously I'm not talking about the scenario where the railcard has expired but not the discount because that would be the same problem as having discounted rail tickets without a valid railcard.