Union Pacific at one time owned 105 Challenger locomotives. When watching the approaching locomotive go through a curve, you can see the boiler swing out left or right independently of the lower half of the engine, as the rear half of the locomotive remains in a straight direction until its wheels and frame are halfway through the curve. The frame of the locomotive is "articulated," or hinged, to allow it to go through curves. In essence, the result is two engines under one boiler. Each set of driving wheels has its own steam cylinder. 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.
The name Challenger was given to steam locomotives with a 4-6-6-4 wheel arrangement. A group of Union Pacific employees volunteered their services to restore the locomotive to running condition in 1981 until it's second retirement. It was retired in 1962 and stored in the roundhouse in Cheyenne, Wyoming, until 1975 when it was placed on display near the Cheyenne depot. 3985 last operated in "regular" train service in 1957. That title has transitioned to Big Boy No. It is one of 105 Challengers built for Union Pacific between 19 and was, for many years, the only operating engine of its class in the world – the largest and most powerful operating steam locomotive. 3985 was designed by Union Pacific and built in 1943 by the American Locomotive Company.