I don't know about the D&RG however from a 1916 C&S Rules and Rates of Pay, I find.
Engineer Per day from Denver to Como
Pass $7.70
Frt $7.28
Fireman Per day from Denver to Como
Pass $5.48
Frt $5.72
So the Engineer makes more on Passenger and the Fireman makes more on Freight. I would GUESS that the fireman has to work more on the freight than the passenger run.
Ken Martin