I seem to recall reading that some tickets "worked the barriers" - now that is a technical challenge, but not too hard for the determined forger with the right skills. One case is mentioned here http://www.newsshopper.co.uk/news/1...d_for_part_in___1_million_train_ticket_fraud/ and has its own forum thread here http://www.railforums.co.uk/showthread.php?t=100605
I don't think working the barriers is too hard. It's just a paper magstrip, and I think you can just copy data from a real ticket, they're not verified online so you can reuse the same data every time.