The Union Pacific, working with the American Locomotive Company, was the inventor of the "Challenger" or 4-6-6-4 wheel arrangement, the first group of fifteen of this type being delivered in 1936. Above, No. 3902 (later renumbered to 3802) poses for the builder's photo from the collection of Wayne Koch. Unusual among articulated locomotives, these first Challengers featured a headlight mounted on the front of the smokebox; most later examples placed the headlight on the pilot so it could swing with the pivoting front "engine." This class CSA-1 had a total weight, including tender, of 889,550 pounds and produced 97,305 pounds of tractive effort. Their four cylinders measured 22x32 inches and their driver diameter 69 inches, and they sustained 255 p.s.i. of boiler pressure.

This modern design had integral cast-steel bed frames and roller bearings on all axles. 108 square feet of grate area, 5304 square feet of evaporative heating surface, and 1650 square feet of superheating surface resulted in the generous steaming capacity for which the Challengers are famous. Intended originally for fast freight service, the Challengers also saw service on passenger trains, including one named for the type, the Challenger. Surviving Challenger 3985 is active today in the Union Pacific Steam Program, being the world's largest operational steam locomotive.