Union Pacific Big Boy 4004

Union Pacific Big Boy 4004
electric trains
Image by cliff1066™
The Big Boys rendered important service in the Second World War, especially since they proved so easy to fire that even a novice could do a fair job. During the war, after German agents filed reports that the Americans had giant steam engines that were moving huge trains full of vital war material over steep mountain grades at high speed, their reports were dismissed as "impossible". Their performance in moving a huge volume of war material throughout WWII was repeatedly cited and the Big Boys are generally acclaimed as having made a huge contribution to the war effort.

Union Pacific Big Boy 4004

Union Pacific Big Boy 4004
electric trains
Image by cliff1066™
The Big Boys were specifically designed to meet the need to pull a 3,600 short ton (3300 metric ton) freight train over the long 1.14% grade of the Wasatch Mountains in Utah and Wyoming. Helpers were needed for this grade at the time. Adding and removing helpers from a train slowed down the movement of trains. For such locomotives to be worthwhile, they had to be faster and more powerful than the slow mountain luggers like the earlier compound 2-8-8-0s and 2-8-8-2s UP tried after World War I. To avoid locomotive changes, the new class would need to pull long trains at sustained speed — 60 mph (100 km/h) — once past the mountain grades.

Union Pacific Big Boy 4004

Union Pacific Big Boy 4004
electric trains
Image by cliff1066™
Helpers were needed for this grade at the time. Adding and removing helpers from a train slowed down the movement of trains. For such locomotives to be worthwhile, they had to be faster and more powerful than the slow mountain luggers like the earlier compound 2-8-8-0s and 2-8-8-2s UP tried after World War I. To avoid locomotive changes, the new class would need to pull long trains at sustained speed — 60 mph (100 km/h) — once past the mountain grades. The Big Boys were designed for stability at 80 mph (130 km/h), so they were built with a heavy margin of reliability and safety, as they normally operated well below that speed in freight service. Optimal horsepower was achieved at about 35 mph; optimal tractive effort, at about 10 mph. Few previous articulated locomotives were capable of such speed, as were UP’s earlier Challenger 4-6-6-4s. In many respects the Big Boy could be regarded as a longer, heavier and more powerful Challenger.

Union Pacific Big Boy 4004

Union Pacific Big Boy 4004
electric train sets
Image by cliff1066™
A Big Boy could generate a maximum of 6,290 drawbar horsepower. The Big Boys were the only locomotives to have the 4-8-8-4 wheel arrangement in the Whyte notation, combining two sets of eight driving wheels with both a four-wheel leading truck for stability entering curves and a four-wheel trailing truck to support the large firebox. The Big Boys were specifically designed to meet the need to pull a 3,600 short ton (3300 metric ton) freight train over the long 1.14% grade of the Wasatch Mountains in Utah and Wyoming.