In my experience, unless it's a big bus operation with RPIs present, there are no ticket checks on RRBs, as the drivers have no training in rail tickets or their validity, and no means to issue tickets to anyone without one.
I remember being on an RRB from Llanwrtyd to Swansea on the Heart of Wales line, arranged at short notice after a train failure. At every stop passengers were offering to pay the driver, who had to reiterate each time that he had no knowledge of train fares nor any apparatus to issue tickets. This is a line with very few ticket offices or TVMs, where many passengers buy on the day from the conductor, so most people travelled free that day.