ASME-Landmark:Union Pacific Big Boy 4023 and Centennial 6900


To pull heavy freight trains on fast schedules over long distances and mountain grades, the Union Pacific railroad purchased some of the largest steam and diesel-electric locomotives ever built. No. 4023 is one of twenty-five "Big Boy" articulated steam engines that operated between 1941 and 1959. It was specifically designed to haul fast, heavy eastbound freight trains between Utah and Wyoming, over a 1.14 percent grade. All of the Big Boys were coal-burning, stoker-fired, and designed to run 7,000 horsepower at 70 miles per hour. They have been lauded in the industry as the highest horsepower, heaviest, and longest steam locomotives ever built. After the war, the Big Boy locomotives were slowly superseded on the original assignment by diesel locomotives, finally ending their operation in 1959.

No. 6900 was the first of forty-seven 6600-hp "Centennial" diesel-electrics, which saw service from 1969 to 1984. Like the Big Boys, 70 mph was attainable with heavy tonnage trains on level track. These locomotives were equipped with speed recorders, dynamic braking, cab signals, and turbochargers. Unlike the Big Boys, the Centennials were designed to operate over all the Union Pacific main lines. And while the Big Boys were assigned to passenger moves such as troop trains, the Centennials arrived about the time passenger service was ending.

While both designs were unique to the Union Pacific, they incorporated many of the best features of other contemporary American locomotives. See ASME website for more information