That is a great find, and good quality images too. It is always a tantalizing challenge to try to unravel the mystery of unidentified, un-located, undated photos, and those photos surely are good candidates for the challenge.
I wonder if that cut they are making exists today. The trees look like a mix of conifer and poplar similar to northern Minnesota, Michigan, or Wisconsin. I am not sure if that forest pattern is typical of western states or not. Perhaps it is, but others may know for sure.
Looks like they have at least two dirt trains in operation. The steam shovel might offer a lot of clues as to the date. The name of the company on the dump cars is a big clue, and probably could be found in other records.
They appear to be making a fill by the use of a temporary trestle to carry the dump cars. Generally, the use of a steam shovel, narrow gage dump car trains, and temporary trestles to make fills was state of the art for the second round of railroad construction around the late 1800 era and later. It was common for improvement projects to railroads that had been originally excavated with horses, scrapers, and handwork in earlier periods such as circa 1870. But that second round of improvement often employed steam saddle tank switchers for the hauling. So the use of horses in conjunction with a steam shovel might be a clue.
That is wonderful detail of the two horses in the close shot, and those other two horses in the first photo sure know how to pose for a photograph. Those guys and their hats seem to show up in every photograph of railroad construction in the late 1800s.