At the moment a rail franchise operator is able to charge a penalty fare if you don't have a ticket and cannot prove that the you had to wait in excess of a given period of time for a ticket. The burden of proof is therefore placed on the consumer and in practice it is impossible to proove this. This is like saying that you can be prosecuted for shop lifting by taking something off the supermarket shelf BEFORE you leave the store.
This should be changed so that as long as the consumer has made an effort to pay, no penalty will be applied. Walking through the ticket barrier without a ticket would obviously trigger this situation – just the same as walking out of a shop with an unpaid for item does the same.