To address this problem, UP worked with the American Locomotive Company to design a new engine. They took the largest steam locomotive that ALCO had, the 4-12-2, and started reworking it. The biggest change was to split the 12 wheels into two groups of 6 wheels. The front 6 drivers and 4 pilot wheels would be on a sub-frame that was able to swivel independent of the rest of the locomotive. This is a feature called articulation. Other changes included going from 3 cylinders to 4 cylinders, a larger firebox, larger boiler, and slightly higher steam pressure.
The resulting machine was a winner. UP tested the new 4-6-6-4 engine on a section of track in Utah over the Wahsatch Range. The engineer commented that would be a challenge for any locomotive, and the name stuck. UP officially called the machine the Challenger.
A total of 252 Challengers were built between 1935 and 1943. UP received the most, 105. Other railroads that used the Challenger include the Clinchfield, Delaware & Hudson, Denver & Rio Grande Western, Great Northern, Northern Pacific, Spokane Portland & Seattle, the Western Maryland, and the Western Pacific.
As WWII progressed, the Challenger and its brother the Bigboy, were workhorses moving war materials across the US. But as soon as the war ended, diesel locomotives started becoming available. The diesel locomotives were so much more efficient that the steam machines were quickly doomed. The Challengers started being retired in the late 1950s, some after only 10 years of service. All were gone from UP service by 1962. Of those 252, only 2 survive today. UP #3977 is on static display, and UP #3985 remains in active service with the UP Heritage Fleet.
