They're often proclaimed as the largest steam locomotives ever built, but that title is quite controversial - there were heavier locomotives, and possibly more powerful locomotives. However, without tender, the Big Boy's locomotive body was the longest of all of them and fully loaded with water and fuel the Big Boy was the heaviest of all of them, even though the locomotive without tender was lighter than some. However, the Big Boy is in the running in every category, and it certainly could be said to be the most successful of the giant locomotives.
The Big Boys were certainly the only locomotives to have the 4-8-8-4 wheel arrangement in the Whyte notation - in other words, combining two sets of eight driving wheels with both a four-wheel leading truck for stability at speed and a four-wheel trailing truck to support a large firebox. Just examining the locomotive arrangement makes it clear that the Big Boy's forté was power at speed, and that's exactly what they were designed for.
The Big Boy locomotives were created by the Union Pacific's need for a locomotive that could pull a 3600 ton freight train over the long 1.14% grade of the Wasatch. Helpers were needed for this grade at the time, but adding and removing them, crewing them, etc. both slowed down the movement of trains. However, for such a locomotive to be worthwhile, it had to be more than just a slow mountain lugger; to avoid locomotive changes, the new class would have to be able to pull that long train at speed - 60mph - once past the mountain grades.
In fact, the Big Boys were designed to be stable at 80mph, so they were built with a heavy margin of safety. Few previous articulated locomotives were capable of such speed; UP's earlier Challenger 4-6-6-4s were, however, and in many respects the Big Boy could be regarded as a longer, heavier and more powerful Challenger.
Twenty-five of them were built, split into two groups of 20 locomotives and 5 locomotives respectively. All were coal burners, with large grates to burn the Union Pacific's low quality Wyoming coal. One locomotive, #4005, was temporarily converted to oil firing, but unlike the experiences on the smaller Challengers, oil firing was not successful, and the locomotive soon reverted to standard configuration.
They did sterling 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 men who were unsuited to combat service were instead drafted into railroad service to replace crewmen who joined up, this proved essential.
Like all steam locomotives, postwar increases in the price of both coal and labor meant that the writing was on the wall, but even so they were among the last steam locomotives taken out of service. The last service train hauled by a Big Boy was in July, 1959; most were stored operational until 1961, and four remained in operational condition at Green River, Wyoming until 1962.
Fortunately, the Big Boy is one of the best represented of preserved steam locomotives in the United States; its fame, and its more western area of operations, saw to that (towns and museums in western states tended to have more room to store such a massive gift). Eight of the twenty-five still exist:
There are currently no serviceable Big Boys and no plans to restore one to operating condition. It's the general wisdom that, short of some very rich individual or corporation deciding to spend several million dollars on the task, that it will never happen in the forseeable future. The most likely corporate sponsor would be the Union Pacific, but they already have enough operational historical locomotives, including UP 3985, a Challenger, the Big Boy's slightly smaller cousin. Since the two look very similar, it's unlikely the railroad would want a bigger, harder to handle, more expensive to run locomotive.
Even if one were restored, the size and weight of a Big Boy mean it would be very hard to find anywhere to run one. The giants of steam produced near the end of the steam age were not general purpose machines; they could not roam whole railway systems, but were generally confined to certain routes where their size and weight were acceptable.
Statistics
Overall length: 132 feet 9¼ inches
Total weight with tender: 1,200,000 lbs approx
Weight on drivers: 540,000 lbs
Tractive effort: 135,375 lbs
Cylinder dimensions: 23¾ inches diameter × 32 inches stroke (4 cylinders)
Boiler pressure: 300 pounds per square inch
Driving wheel diameter: 68 inches
Tender coal capacity: 28 tons
Tender water capacity: 24,000 gallons
Top speed: 80 mph