Until I think the 1970s main line guards didn't do "revenue duties", which were done by a separate grade of Travelling Ticket Inspector. These worked to a fixed "beat" on long distance services - the sole Liverpool to Plymouth day train I recollect was always inspected between Shrewsbury and Hereford. Anyone sat incorrectly in the Firsts was just surcharged (not unknown with standees in the seconds). On lesser services passengers in non-corridor First stock might be suddenly surprised by a collector at the window from a mid-journey platform. On Taunton-Minehead non-corridor steam stock, where First passengers were pretty unknown, I recall the First doors were kept locked unless someone could be seen trying them, when the guard would come forward, check tickets unless they knew them as a regular, and unlock (both sides, by leaning out of the opposite window).