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. Previous articulated locomotives had been limited to slow speeds by their design. Technical breakthroughs allowed the UP Challengers to operate with 280 lbf/in²(1.93 MPa) boiler pressure, something usually reserved for passenger locomotives like the FEF Series. They had 69-inch (1,800 mm) 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 60 miles per hour (100 km/h), while unheard-of on other railroads using articulated steam locomotives, became commonplace on the Union Pacific.