I mean would you find it justified?
We have fraud prevention training at my workplace. One thing highlighted there is that you cannot commit fraud accidentally, it ought to be intentional. The railway does not need to prove the intend.
Yesterday, I was travelling Shalford - Gatwick airport - Guildford to meet a friend. I bought a return e-ticket Guildford - Gatwick Airport while waiting on a platform at Shalford. It then occurred to be that the purchase was finalised after the train left Guildford. Was I trying to evade fare? Absolutely not. Did I have a valid ticket? Yes, always. However, if there was a ticket check, the officer could easily spot that the ticket was bought after departure...