While it doesn't operate, it's still amazing, and you wonder what it was like when these locomotovies made their way across the country.
Below is the story from the Steamtown web site.
The Big Boys were built for power. They did the work of three smaller engines, pulling 120-car, 3800 ton freight trains at forty miles per hour in the mountains of Utah and Wyoming.
With power, though, comes weight - larger cylinders, pistons, drive rods, boiler and firebox. Steam locomotive manufacturers added more wheels with idlers and powered drive wheels.
The extra wheels added length. Long engines had difficulty squeezing through the sharp track curves, especially in the mountains. A Swiss designer, Anatole Mallet (1837-1919) added a "hinge" to the middle of a locomotive to allow it to "flex" slightly. Two pairs of cylinders supplied power to the two sets of drive wheels.
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.
The 25 Big Boys were built to pull long, fast freight trains over the Wasatch Mountains of Utah and Sherman Hill in Wyoming. They served there until 1959 when the new diesel-electric locomotives took over. The Big Boys were not the most powerful engines, though they were the heaviest. But no engine ever came close to matching Big Boy's combination of speed, power and agility. Today, the Union Pacific "Big Boy" #4012 is preserved and on display at Steamtown. Though it does not operate, it remains a most impressive machine.