It's a well known fact that ticket barriers reduce ticket checks on trains and here's why:
Before Barriers:
Passengers join trains, some with and some without tickets. Guards sell tickets to those without thereby making some money, making the effort seem worthwhile, and gaining them commision.
After Barriers:
Passengers join trains, but (in theory) all with have a ticket of some sorts. Guard will sell very few if any tickets and wonder why he is bothering if everyone already has a ticket. Passengers realise that they have to have a ticket after all so they buy a short distance single or press railcard discount on self service machine which has recently been installed as part of the barrier plan. The only people which require the Guard to do something will be the fare dodgers who have brought a short distance ticket / no railcard etc and are trying it on. These are generally awkward to deal with, result in an argument or lots of paperwork of some sort or both. Guard takes very few £££'s but has had to deal with lots of scroats who are trying it on. Guard wonders why he has bothered and gets ****ed off. Guard decides it is far easier just to sit on a**e in back cab / guards van as everyone has a ticket except the odd scroat but he can't be arsed with arguing with them.
Net Result : Guard has same amount of **** to deal with, but has none of the easy stuff which results in them taking money. So they stay put and do nothing.
I have to say, provided you can put up with the arguments and the hassle barriers actually have advantages. The barriers at Leeds have been great as ticket sales for me have increased - but nearly all are now XS fares or new tickets for not having a railcard. Why? Because honest people who may have paid on the train before quite happily now go and get their ticket at the office or machine first. The dishonest types who want to try it on go to the self service machines, select Railcard discount (when they don't have one), choose "WYPTE Concession Single 35p" when they are not entitled to, or buy a single to Burley Park / Crossgates / Cottingley / Bramley and try and use it to get further- because they now have to have a ticket of some sorts they get a ticket, just not a valid one! Provided you don't mind dealing with the aforementioned scroats it is ok, but if you just like the easy tickets and leave the hard ones (which sadly some people do) then you are stuffed.