Consumer law - and reasonableness - do not trump anything. The legal position (rather than the moral or ideal one) means there are major differences between the railway situation and the other analogies in this thread.
Parliament has permitted the railway companies and ministers to make certain laws and those make certain behaviour a crime even when that seems entirely unreasonable. It would be extremely difficult to argue that permission to cross a gate line or selling a ticket away from a train is permission to travel on a particular train without a valid ticket - even though it would be reasonable for some people to interpret it in that way.
I don’t think that’s a satisfactory way for the law to operate - and I think it’s poor when train companies use their rights in circumstances which are of dubious reasonableness - but that doesn’t change what those rights are.
Using a voucher or being let into an event in irregular circumstances isn’t a crime unless there is dishonesty, because of how the relevant laws have been created. Boarding a train without a valid ticket is, in the circumstances of this thread, a crime regardless of the passenger’s knowledge, belief, or intent.