Looking at the cars, 6 sidegrabs of the drop end variety and pressed steel ends are usually found on the SG Boxcars.
FWIW Dan, there are numerous photos of Flatcars loaded with sacked Gilsonite, those sacks in Bruce's photo are paper bag, normally associated with the 40's or 50's so that warehouse is most likely at the nearest railhead most likely Grand Juction or Craig not at Bonanza and after the abandonment of the URly. The D&RGW 1951 Freight Traffic Guide has (strangely) no listings for either Craig or Grand Junction nor the Bender mentioned Gilsonite refinery.