Numerous times I have been on a train , a guard has discovered someone has no ticket or the wrong ticket and nothing has been done.
Examples that stand out in my mind include a 20 ish year old men who when asked simply said " I ain`t got one, what are you going to do about it" and the guard just walked on and continued checking other people`s tickets ( South Eastern) ; or on the other extreme, an attractive young lady who was on a peak hour train with an off peak ticket despite numerous announcements that it was a peak time train on the platform and the train. ( GWR) " Oh, I didn`t realise" and she was simply let off and not charged an excess fare. Having spent £30 more than her buying a peak ticket just to get that train instead of one that was £30 cheaper 20 minutes later ( I had to arrive at a specific time ) and being an unattractive old bloke, I cheekily asked the guard " would you have let me off if I had not had a peak ticket? " He didn`t reply.
How do you know nothing was done? I don't engage in slanging matches with belligerents who won't cooperate but I do report them to the BTP, who sometimes turn up unexpectedly.