Guards who sell tickets on a Penalty Fare train are supposed to issue a printed Penalty Fare warning to the passenger and draw their attention to that warning. If that's not happening then there may be a training need.
I bought my tickets on the train many times, I witnessed many people purchasing tickets there as well. I have never seen any guards issuing a penalty fare warning.
I think it would save a lot (no printed papers needed, no trees killed) if the guard verbally warns the passenger that it is not ok and it is a byelaws offence.
Of course, it wouldn't make everyone buy the tickets at the station but this would be the first step to reduce the number of tickets sold on the train (for no valid reason) by 20%. (Well, I don't know the statistics but to me, 20% seems to be a good number

)
The training can be done on a local level by just simply telling the guards that this procedure must be introduced.
I think, it just never really been emphasised that the conductors need to do it (just guessing here... by looking at it, it doesn't seem to be a practice)
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It was a suggestion/example, I didn't say it was necessarily what they should run with. If the TOCs are the ones losing money maybe they shoukd be coming up with some other reasonable ideas.
Paper tickets, the way they are, are clearly not working though. This is shown by the need for this sub-forum.
I think paper tickets work fine 99% of the time. For the 1%, we can't expect the TOCs to change the whole system.
I don't know the right answer here either to be honest... It is just an awful lot of money to change the whole system nationwide.