If you board a Southern train without a ticket you may be aware of notices warning about their penalty fare scheme which is £20 or twice the single fare to the next station, whichever is more expensive. Now, if you board at South Croydon (say) and get off at Norwood Junction you'll be greeted by posters explaining that the TfL penalty fare scheme is £50. Which fare would you be charged in this situation? You'll have travelled on a Southern train but allighted at a station operated by London Overground.