46 CLASSIC TRAINS FALL 2019
NO RAILROAD WAS BETTER EQUIPPED TO SHOULDERthe
heavy burden of wartime traffic than Union Pacific, thanks to the three
machines that constituted its front line of defense: the 4-6-6-4 Chal-
lenger, arguably the most successful simple articulated ever made; the
4-8-8-4 Big Boy, which easily wore the mantle “world’s largest steam lo-
comotive”; and the 800-class 4-8-4s, among the finest of all Northerns.
UP already had long experience with the Challenger, introduced in
1936 to answer the railroad’s need for horsepower rather than just pull-
ing power. Railroading’s drag-freight era was over; speed was of the es-
sence. The UP’s reigning giant, the 4-12-2, would give way to the 4-6-6-4,
with its more flexible wheelbase, lighter-weight rods, and ample boiler.
The railroad already had 85 Challengers on the roster in 1944 when it
went back to Alco for 20 more, this time for engines with slightly smaller
cylinders and a shorter firebox but a boost in boiler pressure from 255
to 280 psi. They were the last of UP’s Challengers.
By 1944, the railroad world had already run out of superlatives for UP’s