There is no evidence that 'rounder' as applied to railway workers originated as a name for workers specifically in the roundhouse. It was applied to workers that were moved, or moved, from job to job (making the rounds, as we say today).

Cluin, that is another shade of the meanings that have come about since the term originated.
Actors had another definition; in the sense of a traveling performer.

(Roundhouse is a very old name for circular structures, Its application to a circular shed with a turntable is American. That the railroad term was in existence very early (ca. 1850) is evident from a proposal for a cheaper alternative, without cover, which was made in 1856).