Design
Union Pacific Railroad (UP) introduced the Challenger-type (4-6-6-4) locomotives in 1936 on its main line across Wyoming. For most of the way, the maximum grade is 0.82% in either direction, but the climb eastward from Ogden, Utah, into the Wasatch Range (Wahsatch, on the railroad) reached 1.14%. Hauling a 3,600-short ton (3,300 t) freight train demanded doubleheading and helper operations, and adding and removing the helper engines from a train slowed operations.
The answer was to design a new locomotive, but for such locomotives to be worthwhile they had to be faster and more powerful than slow mountain luggers like the earlier compound 2-8-8-0s that UP tried after World War I. To avoid locomotive changes, the new class would need to pull long trains at sustained speed—60 miles per hour (100 km/h)—once past the mountain grades. (The 1950s Wyoming Div timetables allowed them 50 mph or less, passenger or freight.)
Led by Otto Jabelmann, the UP's design team worked with Alco to re-examine the Challengers, which had been designed by A.H. Fetters. They found that the goals could be achieved by making several changes to the Challenger design, including enlarging the firebox to about 235 by 96 inches (6.0 m × 2.4 m) (about 155 sq ft/14.4 m2), lengthening the boiler, adding four driving wheels and reducing the size of the driving wheels from 69 to 68 in (1.753 to 1.727 m).
The Big Boys are articulated, like the Mallet locomotive design. They were designed for stability at 60 miles per hour (100 km/h). They were built with a heavy margin of reliability and safety, as they normally operated well below that speed in freight service. Peak horsepower was reached at about 35 mph (56 km/h); optimal tractive effort, at about 10 mph (16 km/h).
Without the tender, the Big Boy had the largest engine body of all reciprocating steam locomotives.
Read more about this topic: Union Pacific Big Boy
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