I do take exception to people saying on board enforcement will lead to bad publicity etc. The vast majority pay and buy before they board. If those who don't are taken to account, then so much better for everyone and cheaper fares in the long run, good PR in fact.
When the conductor comes around on my route it's always the same destination for anyone without a ticket "a single to Grangetown please" i.e. they were going between two stations close together without barriers and the conductor sometimes doesn't make it to the front of the train to check all the tickets. So not they forgot, but are trying to avoid paying.
I'd agree if I thought the vast majority buying on board were attempting to fare evade. Perhaps it varies by route - it's certainly not my experience when I'm travelling with TfW. For a start I mostly see people buying tickets to Queen Street or Central.
And I don't think they forgot to buy beforehand either, they've just carried on buying on board as they did before there were station ticket machines and nobody has ever told them to stop. And yes there are signs, though the ones that went up when the ticket machines appeared did not make it clear at all that the presence of a machine meant you had to use it. And if guards carry on selling on trains as before, it's entirely reasonable to think it's OK to do so.
Yes I want to see the fare evaders stopped, and I would be happy to see Penalty Fares issued on board.
But NOT without giving fair warning to honest passengers. Handing out a ticket one day and a penalty fare the next without any warning is not reasonable.
But I think any further discussion probably ought to be in a different thread.
(Added - and since most ticket machines only take cards and therefore despite the signs to the contrary at stations it's permissible to pay cash on board, enforcement would be rather difficult anyway).