Evidence that a person has purchased a ticket should be an absolute defense to any prosecution that rests on the basis that they couldn't present a ticket. This entirely solves the "I left my ticket at home", "my phone's broken", etc, problem. If the railways discover that it's too easy to provide a defense that the prosecutor can't successfully attack... perhaps they need to upgrade their ticketing standards. Maybe with this new-fangled e-ticketing system they've heard so much about.