The Union Pacific Challengers are a type of simple articulated 4-6-6-4 steam locomotive built by American Locomotive Company (ALCO) from 1936 to 1944 and operated by the Union Pacific Railroad until the late 1950s. A total of 105 Challengers were built in five classes. They were nearly long and weighed 537 short tons (487 tonnes). They operated over most of the Union Pacific system, primarily in freight service, but a few were assigned to the Portland Rose and other passenger trains. Their design and operating experience shaped the design of the Big Boy locomotive type, which in turn shaped the design of the last three orders of Challengers. Two Union Pacific Challengers survive. The most notable is Union Pacific No. 3985, which was restored by the Union Pacific in 1981, then operated in excursion service as part of its heritage fleet program. Mechanical problems forced it out of service in October 2010; it was retired in January 2020 after the restoration of the 4-8-8-4 Big Boy 4014 and eventually donated to the Railroading Heritage of Midwest America in 2022, where the locomotive is undergoing a second restoration. The only other surviving Challenger is UP No. 3977, which is on display in North Platte, Nebraska. History Description The name "Challenger" was given to steam locomotives with a 4-6-6-4 wheel arrangement: four wheels in the leading pilot truck to guide the locomotive into curves, two sets of six driving wheels, and four trailing wheels to support the rear of the engine and its massive firebox. Each set of driving wheels is driven by two steam cylinders. In essence, the result is two engines under one boiler. Union Pacific developed five types of Challengers: the "light" CSA-1 and CSA-2 classes and the "heavy" 4664–3, 4664–4, and 4664-5 classes. The railroad sought powerful locomotives that could handle mountain grades at high speeds. Previously, articulated locomotives had been limited to slow speeds by their design. Technical breakthroughs allowed the UP Challengers to operate with boiler pressure, something usually reserved for passenger locomotives like the FEF Series. They had drivers, mammoth wheels usually seen on passenger locomotives only because freight engines normally require the extra torque provided by smaller wheels. Speeds in excess of , while unheard-of on most other railroads using articulated steam locomotives, became commonplace on the Union Pacific. When the first Challengers entered service in 1936, on the UP's main line over the