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The Big Boys were built in Schenectady, New York by the American Locomotive Company (ALCo) to the Union Pacific's design. ALCo delivered the first batch of 20 - including #4012 in the Steamtown NHS collection - in 1941 and the remaining 5 in 1944. Big Boys had over one mile of tubes and flues inside the boiler. Their firebox grate measured 150 square feet. The Big Boys had sixteen drive wheels, each measuring 68 inches. From coupler to coupler they measured 132 feet 9 inches. The tender held 24,000 gallons of water and 28 tons of coal and the engine and tender weighed 1,189,500 pounds in working order. The engines well deserved the name 'Big Boy' which was written on one of the drive rods by an unknown worker at ALCo.