|Big Boy was the name given to the Union Pacific Railroad's twenty-five 4000 class 4-8-8-4 Mallet articulated steam locomotives built between 1941 and 1944 by Alco.|
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. 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 Boy locomotive was articulated, per the Mallet locomotive design, but used simple (single) rather than double expansion, unlike the original Mallet design.
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.
In total, 25 Big Boys were built, in two groups of ten and five locomotives. All were coal burning, with large grates to burn low quality Wyoming coal from mines owned by the railroad. One locomotive, #4005, was experimentally converted to oil burning. Unlike experience with the Challenger types, this change was not successful and the locomotive soon reverted to coal. The cited reason for this failure was the use of a single burner, which, with the Big Boy's larger firebox, created unsatisfactory and uneven heating. It is unknown why multiple burners were not employed, though with dieselization in full swing after 1945 the company probably lost interest in further development of steam.
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. Since many new men who were unsuited to combat service or exempted were hired by the railroads to replace crewmen who had gone to war, this proved advantageous. 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.
Postwar increases in the price of both coal and labour and the efficiency of diesel-electric and gas-turbine motive power foretold a limited life for the Big Boys, but they were among the last steam locomotives taken out of service. The last revenue train hauled by a Big Boy was in July 1959, the last run ending early in the morning of July 21st. Most were stored operational until 1961, and four remained in operational condition at Green River, Wyoming until 1962. Their duties were gradually taken over by diesels and turbine locomotives.