It is a colloquialism for someone who is among the group of individuals who have had the most mileage off a class of engines, or the largest distinct number of locomotives from a class.“Top men” sounds like a descriptor for a group of football “enthusiasts” and not something I’ve heard in a rail context![]()
I'm not sure how the poster using the phrase has identified that there weren't any "top men" on the train, other than perhaps identifying that the "top men" for HST haulage are not those who are "top men" for other classes.