Thanks for the tip. It looks like the image the DPL has is a photo of an original watercolor by Graham. (One wonders where the original painting might be!)
I was able to find the accompanying article on the Hathi Trust archive. It's "A Picturesque American Railway" by one William Willard Howard, in the Feb. 4, 1888 issue. Although the specific location of the illustration is never identified, the article does focus on the Marshall Pass main line and mentions the Pacific Express, so Marshall Pass does seem likely.
Here is the first part of the article:
(To be continued.)
-Philip Marshall