Given the link-n-pin couplers on the boxcars, I would date the photo as between 1899 and 1903.
And the photo illustrates two rare early C&S boxcars:
Boxcar 7731 illustrates one of the first order of St Charles 30' boxcars, ordered by the receiver in 1897, delivered in early 1898, originally lettered for the UPD&G, 6500-6524. With the creation of the new corporate C&S in the first days of 1899, these new cars were considered to be "inherited" and were repainted and lettered with the new "Colorado Road" billboard lettering, numbered 7722-7746. Here is how one of them looked at the end of operations, on the clean up train:
The boxcar to the left, 7682, is one of the 30' Peninsular boxcars built for the DSP&P in about 1884.
Both boxcars have atypical lettering for the period, showing that, in haste to repaint the freight car fleet, there was a lot of deviation from standards. 7682 has the "Colorado road" herald on the door of the boxcar and the car number is located lower on the right car side than most. Car 7731 has no box herald at all. Note the end lettering has C&S to left and number to right.
Thanks for posting this beautiful photo, Jeff. It will be of great help with my 1901 roster. I have a 7700 series St Charles boxcar under construction -- now I know how to letter it!