After the success of the Challenger, the Union Pacific planned to have only one locomotive haul express freight trains on Wasatch Grade and Sherman Hill in the future. To haul trains of 3,600 short tons and over it was calculated that an adhesive mass of 540,000 pounds would be required. The grate area should be large enough to be able to develop high power even with inferior coal. This required eight coupled axles and also four carrying axles in order to be able to carry a sufficiently strong boiler. At the same time, the locomotives should be designed for a speed of 80 mph. 

The locomotive developed in this way was the heaviest, but not the most powerful, conventional steam locomotive of all time. Like most modern American articulated locomotives, they had four cylinders with simple steam expansion and were therefore no true Mallets. The running numbers started from 4000, in fact the class was given the designation 4884 due to the lack of a name for the wheel arrangement. The Union Pacific first wanted to officially call it “Wasatch”, but the unofficial name “Big Boy” prevailed. In 1941, 20 class 4884-1 locomotives were built and in 1944 five more class 4884-2 locomotives.

In order to allow encounters between two Big Boys, the gap between the tracks had to be increased in many curves. The chassis had no problems with tight corners. In practice, they also pulled trains of 6,000 short tons and often reached their permitted speed of 70 mph with lighter trains. The number 4005 was converted to oil firing, but was converted back to coal firing after a short time. They later received an extension on the tender that increased capacity from 28 to 32 short tons. 

In the 1950s, the Big Boys were replaced by the gas turbine locomotives. The last commercial journey with a freight train took place in 1959. The 20 locomotives of the first series all achieved a mileage of just over a million miles. A total of eight Big Boys still exist today. The 4014 was the only one to be rebuilt and has been operational again since 2019. This makes it the most powerful operational steam locomotive today.