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The "Challenger" of the Union Pacific The "Challenger" steam locomotives were born at the Union Pacific Railroad (UP). In the mid-1930s, UP chief engineer Arthur H. Fetter and Otto Jabelmann (UP Superintendent for locomotives) recognized that heavier train loads at higher speeds could no longer be carried by the existing steam locomotives. In particular, freight transport should be accelerated at the 0.82% gradients in Wyoming. In collaboration with the American Locomotive Company (ALCO), the two UP engineers developed an articulated steam locomotive "Articulated" with the wheelset sequence (2'C) C2'h4. Although it corresponded to the Mallet design, but not work according to the composite principle but with simple steam expansion on each drive wheel. The first prototype with the number 3900 went on 25. August 1936 in Council Bluffs, Iowa, in operation. The locomotive had the latest steam technology of the time, including roller bearings on all running and tender wheelsets, a generously dimensioned boiler and a suitably adapted combustion chamber. With a top speed of 70 mph (112.65 km / h), it had the following main data: Boiler pressure 17.93 atü; four cylinders (558.8 x 812.8 mm); Driving wheel diameter 1,752.6 mm (Boxpokräder); Grate area 10.05 m²; Heating area 499 m²; Superheater 151.1 m²; Weight of the locomotive 256.5 t; Friction weight 175 t; Semi-Vanderbilt tender with two three-axle bogies. Their first test was the covering of a heavy freight train from Ogden (Utah) to Green River (Wyoming), tackling the slopes of the Wasatch Mountains. To test the performance of the machine, UP officials decided to run the locomotive without support. The 3900 made the trip with flying colors and Fetter explained that the new construction would easily have met the challenge. So the name "Challenger" stuck to the locomotive. Up to 1937, the UP then with three deliveries a total of 40 copies with the No. 3900-3939 (from 1944: 3800-3839) in service, the 3934-3939 were rebuilt immediately after delivery on oil firing and more followed in the next few years , In 1942-1944, the UP continued to expand with 65 "Challengers" (3930-3949, 3950-3969 and 3975-3999). With many improvements taken from the "Big Boys" (eg more efficient boiler, roller bearings on all wheelsets, enlarged cab) they now called "heavy" Challenger in contrast to the "light" Challengers 1936/37. In 1945, the 3975-3984 were converted to oil firing, 1952 followed by the 3930/31/32/34/37/38/43/44. They received the new numbers 3700-3707 in the same year, while the 3975-3984 were redrawn in 3708-3717. Initially, the Challenger mainly hauled freight trains across the slopes of the Wasatch Mountains and Sherman Hill, following the intervention of the even more capable Big Boys throughout the UP system in California, Nebraska, Oregon, Utah, and Wyoming. In addition, they were occasionally also found before passenger trains such as the eponymous "Challenger Streamlinern" between Chicago and California. With fast Verdieselung after the Second World War went the last "Challenger" in 1959 from the service.