Thameslink need to be out in force everyday ticketless travel is rife
AIUI 700s are the problem. As explained to me by local staff.
There are too few on train bolt holes for RPIs to dive into when they encounter aggression. On a train composed of say 377s (and once 319s) there are intermediate cabs they can lock themselves in, and such a cab is never more than 2 cars away. On a 700 the free cab may be 11 cars away, as the actual driving cab does not count. And that is a long way to run. I am sure someone is bound to suggest toilets but the staff who told me that had a very good reason why not that I'm not going into here.
Also, the very open wide gangway nature of 700s means that an approaching RPI can be seen a long way off enabling efffers to prepare their story,and on a stopping train often reaches next station before the RPI has got there.
Lastly, sound carries a long way on a 700. RPIs have difficulty in carrying out some of their procedures due to the necessary conversations being overheard hence not being private - especially when summoning assistance. Again, intermediate cabs could be used for this. Not on a 700.
What this boils down to is RPIs are reluctant to do on train check unless in gangs of 4s or with police protection.