as big as they got - Union Pacific's Big Boys

The Big Boys were the only steam locomotives in the world to have a 4-8-8-4 wheel arrangement, combining two sets of eight driving wheels with a four-wheel leading truck for stability entering curves and a four-wheel trailing truck to support the large firebox.

The Big Boys - given a 4000 classification by Union Pacific - were designed to pull a 3,300 tonne freight train over the long 1.14% grade of the Wasatch Mountains in Utah. Before their arrival, helpers were needed. Adding and removing helpers from a train slowed them down. 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 Union Pacific tried after WW1. 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. Towards the end of the 4000's career in the late 1950s it was found that they could still pull more than their rated tonnage. Their ratings were increased several times until they regularly pulled 4,040 tonnes up the Wasatch grade, unassisted.

They were articulated, as per the Mallet locomotive design, but used simple (single) rather than double expansion, unlike the original Mallet design. They were designed for stability at 60 mph and were built with a heavy margin of reliability and safety, as they normally operated well below that speed. Peak horsepower was reached at about 35 mph (56 km/h); optimal tractive effort at about 10 mph (16 km/h).

25 Big Boys were built, in two groups of ten and one of five. 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. Unlike experience with the Challenger types, this was not successful, and the locomotive was soon changed back to coal. The reason given for this failure was the use of a single burner, which, with the Big Boy's large firebox, created unsatisfactory and uneven heating. It is unknown why multiple burners were not employed, though with dieselisation in full swing after 1945 UP probably lost interest in further development of steam.

Postwar increases in the price of both coal and labor and the efficiency of diesel-electric locomotives meant 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 ended its run early in the morning on 21 July 1959. Most were stored operational until 1961, and four remained in operational condition at Green River, Wyoming until 1962. Their duties were assumed by diesels and turbines.  Eight are preserved.