HO 4-6-6-4, D&RGW #3803 Overview
During the World War II industrial production was overseen and regulated by the Federal War Production Board. When the UP placed their 2nd Challenger order with Alco in 1943 the WPB stepped in and had 5 of the locomotives built for the Denver & Rio Grande Western RR to help fulfill their need for additional motive power to haul wartime traffic. These 5 locomotives were built to UP's specifications.
The name "Challenger" was given to steam locomotives with a 4-6-6-4 wheel arrangement. This means that they have four wheels in the leading pilot truck, which helps guide the locomotive into curves, two sets of six driving wheels, and finally four trailing wheels, which support the rear of the engine and its massive firebox. Each set of six driving wheels is driven by two steam cylinders. In essence, the result is two engines under one boiler. The Union Pacific Railroad sponsored development of this type to meet the need for higher speeds in main-line service. Historically, articulated locomotives had been limited to slow speeds by factors inherent in their design. The technical breakthroughs achieved with the Big Boy enabled the carrier to develop a newer, improved Challenger that met their speed expectations.
Though originally intended for freight service, many Challengers were used in passenger service.