I kinda doubt Geese were used to move any revenue frieght cars. First of all, I doubt they could move anything loaded up any kind of hill. Additionally, the geese did not have automatic air brakes, to would have been illegal to use it as a locomotive. The geese were rather light (10 tons?), the idea of a loaded boxcar of zinc concentrate wieghing about 35 tons shoving it down Dallas Divide sounds pretty frightening, especially with all the stories of how marginal the brakes were to begin with.
Another thing is the unions probably would squash it. The RGS had a different pay rates for engineers, conductors and "motormen" (goose runners). If it hauled a car it would become a "train" requiring an engineer and conductor. I could see the local BLF&E chairman going nuts over that....