Wasatch Range between Green River and Ogden, the locomotives had problems climbing the steep grades. For most of the route, the maximum grade is 0.82% in either direction, but the climb eastward from Ogden, into the Wasatch Range, reached 1.14%. Hauling a freight train demanded double heading and helper operations, and adding and removing helper engines slowed operations. Those limitations prompted the introduction of the Big Boy in 1941, as well as a redesign of the last three orders from 1942 to 1944. Using the experience from the Big Boy, UP chief mechanical engineer, Otto Jabelmann, redesigned the last three orders of Challengers in 1941. The result was a locomotive in working order weighing some accompanied by a tender weighing when 2/3 loaded. Calculated tractive effort is . From 1941, the Challengers were intended to speed up freight operations on the grades across Wyoming; the Wasatch Range climb east from Ogden was to be conquered by the Big Boys without helpers. Construction The 105 locomotives were ordered in five batches: the first two of light Challengers, and the final three of heavy Challengers. Along with the Big Boys, the Challengers arrived on the scene just as traffic was surging in preparation for American participation in World War II. As part of Union Pacific's fourth order in 1943, ALCO built thirty-one locomotives for Union Pacific using the same specifications. However, the War Production Board diverted six locomotives after completion to the Denver and Rio Grande Western Railroad via a lease through the War Department's Defense Plant Corporation. Locomotives 3900-3905 formed the Rio Grande's Class L-97. These were later sold to Clinchfield Railroad in 1947 and were renumbered as 670–675, where they formed the Clinchfield's Class E-3. Preservation Only two Challengers survive, both from the 4664-4 order built in 1943. No. 3977 is on static display in North Platte, Nebraska. No. 3985 was restored to operating condition by Union Pacific in 1981 and used in excursion service until mechanical problems sent it back into storage in October 2010. It was officially retired in January 2020 and is stored in Cheyenne, Wyoming, but has since been acquired by the Railroading Heritage of Midwest America (RRHMA) in Little Silvis Shops in Silvis, Illinois, where the locomotive is now being restored to operation once again. See also Union Pacific Big Boy Union Pacific FEF Series References Bibliography Further reading External links Union Pacific Steam Shop