Forney Museum
of Transportation

The 4-8-8-4 class series, originally rumored to be called the "Wasatch", after the Wasatch Mountains, acquired its nickname after an unknown ALCO worker scrawled "Big Boy" in chalk on the front of No. 4000's smokebox door, then under construction as the first of its class.

The Big Boys were articulated, like the Mallet locomotive design, although lacking the compounding of the Mallet. They were built with a wide margin of reliability and safety, and normally operated well below 60 miles per hour in freight service. Peak drawbar horsepower was reached at about 41 mph. The maximum drawbar pull measured during 1943 tests was 138,200 lbf while starting a train.

The Big Boy has the longest engine body of any reciprocating steam locomotive, longer than two 40-foot buses. They were also the heaviest reciprocating steam locomotives ever built; the combined weight of the 772,250 lb engine and 436,500 lb (198,000 kg) tender outweighed a Boeing 747.

The locomotives were held in high regard by crews, who found them sure-footed and more “user friendly” than other motive power. They were capable machines, and their rated hauling tonnage was increased several times over the years. But postwar increases in the price of coal and labor, along with the advent of efficient, cost-effective diesel-electric power, spelled the end of their operational lives. Nonetheless, they were among the last steam locomotives withdrawn from service on the Union Pacific. The last revenue train hauled by a Big Boy ended its run early in the morning on July 21, 1959.

Wikipedia