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 turbines.
It is often stated that the Big Boy was the largest steam locomotive ever built. However, this is heavily debated. For example, weight, length, horsepower and tractive effort are all categories in which a locomotive can be ranked, and in each of them a locomotive "larger" than a Big Boy can be found. However, in overall performance and reliability, among all the "heavy iron" prototypes the Big Boys were unsurpassed.
Without a tender, the Big Boy's locomotive body was the longest engine, although the smaller-diameter boiler of a Big Boy fits inside an H-8 Allegheny's boiler. The H-8 Allegheny was also somewhat heavier than a Big Boy, weighing in at 1,207,040 lbs.
The Big Boy is one of the best represented and preserved model of steam locomotive in the United States, due to its mythical reputation and late survival in service to 1959. Eight of the 25 still exist:
All except numbers 4005 and 4017 are currently stored in the open without protection from the elements. The dry air of Southern California has helped #4014 to remain the best preserved of the survivors, assisted by care of the local chapter of the Railway and Locomotive Historical Society. The Steamtown example is also said to be in good condition, though the harsher weather of the northeast has taken its toll. The Forney Museum of Transportation in Denver moved the 4005 to a renovated building in January 2001. Thanks to considerable fundraising and volunteer efforts, number 4017 now resides with other pieces of railroad equipment in a climate-controlled shed at the museum in Green Bay. Number 4023 is the only known Big Boy to move by highway since preservation, to the new Kenefick Park in Omaha, NE.
There are currently no serviceable Big Boys and no plans to return any to operation due to their size and operating costs. When the locomotives were constructed, changes were made in clearances on the Wasatch main line to accommodate them. In the nearly fifty years since their retirement, these clearances have been altered. There were only a few terminals on the UP with turntables long enough to reverse the Big Boys, and most of those have been removed, although certainly there are many wyes on which a Big Boy could be turned. The coal consumption of these locomotives was prodigious and the only conversion to oil was a failure. Challenger 3985 was converted to oil a few years after its restoration because the sparks it spread caused grass fires. The State of Colorado purported to ban the engine from operation there until the change was made (although such a ban is arguably beyond the power of any state to enforce, being solely within the authority of the Federal Railroad Administration). In the diesel age, diesel fuel is much easier to obtain (and load) than coal, so the surviving Big Boys will probably remain inoperative.
In 2006, a Cheyenne radio station played an April Fool's prank on its listeners by claiming that, during the previous night, the Big Boy locomotive on display in Holliday Park had been stolen. Despite the near-impossibility of the theft of such a heavy item with no tracks connecting it to a railroad, several listeners fell for the joke and went to investigate.
William W. Kratville, "Big Boy", Omaha: Kratville Publications, 1972.