Andy:
If you click on the Denver Public Library Western History Collection link at the bottom of the photo and search for the catalog number, you will find all the information about that photo. In this case, it was taken by Bob Richardson in 1951 and here the caption info:
Three-quarter view of left side of engine, from front end; with 13-car stock train. Photographed: Matterhorn, Colorado, September 25, 1951.
Jerry Day