It isn't really.God im confused why does it need to be so complicated
If there are facilities to sell you a ticket using your chosen valid payment method, then you must use them, unless there are notices or publications saying you can buy on board.
If you do not use them, and it was deemed that you made a mistake or were in a rush etc, then you can be charged the full fare or, if you encounter an authorised person under the relevant circumstances, a Penalty Fare. However the train company may, depending on the circumstances, choose not to issue any fare and instead prosecute you.