The
Union Pacific Challengers were a type of simple articulated
4-6-6-4 steam locomotive built by
American Locomotive Company for the
Union Pacific Railroad. 105 of these locomotives were built between 1936 and 1943. The Challengers were nearly long and weighed 284,800 kg. 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 to California and Oregon. The locomotives were built specifically for
Union Pacific and much of the experience gained later went into the design of the "
Big Boy".