Both the C&NW and the CB&Q had narrow gauge lines in the Black Hills. The high number on the boxcar suggests that it would belong to a large RR, such as the C&NW or CB&Q.
Based on the blunt end of that baggage car or RPO, with the clerestory roof not tapered down, I would guess the CB&Q. I found a photo of a standard gauge CB&Q baggage car with an end just like that - on a train running in the Black Hills to boot. I don't know exactly where the CB&Q narrow gauge and dual gauge lines were in the Black Hills, tho.
Robert