Most powerful steam locomotive in operation

The most powerful steam locomotive currently in operation is the Union Pacific "Big Boy" 4014, a simple articulated 4-8-8-4 locomotive capable of a tractive effort (pulling force) of 135,375 lbf (602 kN) at 10 mph. Number 4014 was built in November 1941 by the American Locomotive Company of Schenectady, New York, for the Union Pacific Railroad Company. The locomotive was in service from December 1941 to July 1959. In 1962 it was transferred to the Rail Giants Train Museum in Los Angeles, California, where it remained on static display until 2012, when 4014 was transferred back to Union Pacific, who restored it to working order and put it back in service pulling excursion trains on 4 May 2019.

The Big Boy series of locomotives were designed to pull 3,600-ton (3,266-tonne) freight trains over the Wasatch Range between Utah and Wyoming. A total of 25 were built between 1941 and 1944. On level ground the Big Boys could reach speeds of up to 70 mph (112 km/h). Initially, the Big Boys were used for the roughly 450 mi (725 km) run from Ogden, Utah, to Cheyenne, Wyoming, but as steam locomotives were gradually phased out in favour of cheaper-to-operate diesel units, the powerful Big Boys were used only for the steepest and most demanding parts of the route.

The locomotive weighs 1,524,000 lb (691,274 kg), plus another 427,000 lb (193,683 kg) for the 14-wheeled tender, and it can carry 25,000 gallons (113,652 litres) of water. It was originally designed to run on coal, but was converted to run on No.5 fuel oil as part of the 2012–2019 restoration. Today the 4014 is used as a sort of mascot for the Union Pacific Railroad Company, travelling around the United States on tours which include carrying paying passengers on excursions and making appearances at regional events.