Oyster cards. Pre-pay. Do you have to somehow specify the journey you will make with them? Say you wanted to go from a One Railways station near Enfield, which doesn't have a Oyster pad, to King's Cross, via Seven Sisters, which do. Here's the scenario: You'd get a Oyster, and charge it up. Then, you'd get on the train to Seven Sisters. At Seven Sisters, you'd do a "fake swipe in", by touching your Oyster card to the pad on the incoming gate. At the moment there's a "people diversion" out onto the street and then back into the station at SS, so this will confused things[1] , but we'll ignore that for now. Then, get the tube to King's Cross, and swipe out as normal. So, if I'm understanding how the Oyster card system works, you'd only be charged for a Zone 1-3 journey, rather than a Zone 1-5 one. The only catch I can see is if there is ever an inspector on the train between SS and the station near Enfield (I've never seen one after SS). Am I completely misunderstanding the Oyster card system. And apart from an analysis of my outlined fraudulent system, what are people who live outside the Oyster network /supposed/ to do? [1] currently, you'd swipe "out" and then back "in" at SS - if your first swipe "out" is on an Underground station, where does the Oyster system assume you'd swipe "in"?