The mid 1920s to the late 1940s. Early on there are still plenty of the smaller classes - T-12, C-16, C-18 and C-19 in regular service; the D&RGW interchanges with a wholly owned subsidiary (RGS) and with the Silverton Northern. By the end of this period there are still some representatives of the smaller classes left, but still a fairly active railroad, though some of the branches and the SN are gone.
I model the D&RGW and RGS in the period above, though I have some equipment that could stretch things to the end and the pipe trains.
Charlie Mutschler