Spokane, Portland & Seattle (SP&S) 903 photographed by Walter E. Frost, on August 2, 1953. Sold to Great Northern (GN) and purchased back. So if you have photos of GN 4000 or 4001 that was SP&S 903 and 904.
Confronted with moving freighters over the Wasatch Mountains at track speed, Union Pacific Railroad designed this wheel arrangement in conjunction with the American Locomotive Company. Click here to learn why this wheel arrangement was dubbed "Challenger." It was a design ordered by several roads.
The Spokane, Portland & Seattle was result of a joint venture between the Great Northern Railroad and the Northern Pacific. In fact in the early years following construction, SP&S was operating with used equipment from both roads.
For more of the roads interesting history, I direct you to the Spokane, Portland & Seattle Railway Historical Society.
The Whyte Notation for this engine is 4-6-6-4. This would be 3/5+3/5 in Switzerland, 230+032 in France.
SP&S 903, road class Z-6, built by American Locomotive Company in 1937.