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Available in both HO and N Scale Athearn's new 4-6-6-4 Challenger Locomotive are the largest steam locomotives still operating anywhere in the world.

The Union Pacific Challengers were 105 simple articulated 4-6-6-4 steam locomotives built by American Locomotive Company for the UP. Built between 1936 and 1943, the Challengers were nearly 122 feet long and weighed more than 1,000,000lbs. They operated over most of the Union Pacific system, primarily in freight service, but a few were assigned to passenger trains operating through mountain territory.

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 driving wheels has its own steam cylinder. In essence, the result is two engines under one boiler.

As part of UP's 4th order in 1943, ALCO built 31 locomotives for UP using the same specifications. However, the War Production Board diverted 6 locomotives to the Denver & Rio Grande Western Railroad via a lease. Locomotives 3800-3805 formed the Rio Grande's Class L-97. These were later sold to Clinchfield Railroad after the war to become #670-675.

Only two examples survive today: Union Pacific 3985, used for excursion services by Union Pacific and Union Pacific 3977, which is on static display in North Platte, NE.

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