After I posted what may seem to some an inflammatory statement, this was confirmed by an incident (or, fortunately, non-incident) which happened to a friend this week, which she mentioned casually and in no connection with these forums or any discussion about rail issues. She had been walking along the canal in North Manchester and took a train into Victoria from a non-staffed station where there was no ticket machine. The guard was clearly occupied with other matters since he didn't bother to come round and sell her a ticket.
At Victoria, the barriers were simply left open (as it was off-peak the hordes of barrier staff which normally outnumber actual rail employees were presumable relaxing somewhere). She simply walked out of the station without paying, as I suggest the vast majority of honest passengers also would, taking the view that Northern are not prepared to put any effort into collecting revenue off-peak. She didn't normally travel by train and may not have realised that in such circumstances the passenger is meant to do the TOCs work for them and actively seek out a ticket office somewhere on the station.
The good news is that there was no RPI lurking behind a pillar ready to criminalise her, but, hey, the fare was probably only around £3 anyway. However the consequences for her could have been serious, since railway law has provisions which would be viewed as inequitable under consumer legislation in any other area.
I recently viewed a site of a solicitor specialising in defending passengers accused of fare evasion - whilst in most cases the issue was clear-cut and involved trying to negotiate a financial settlement as opposed to a conviction which could affect the defendant's career, in a significant minority of cases, maybe 10-20%, the TOC offered no evidence or were disbelieved and the defendant was found innocent and had costs awarded. But these are, I suspect, fairly well-heeled individuals prepared to take the risk of going to court, many passengers will simply fork out meekly and it's noticeable that in these forums at least half the genuine disputes seem to involve Northern.
I doubt whether they care too much, since if they win the passenger pays up or is criminalised, and if they lose the taxpayer pays up. But if the management of TOCs had to face the possibility of a prison sentence for malicious or incompetent prosecution, bearing in mind that the passenger faces this kind of risk habitually, it might concentrate minds wonderfully on proper and fair means of revenue collection.