548 t1,210,000 lb
A Union Pacific Big Boy weighs about 548 tonnes (1,208,750 lb) with its loaded tender. That is the official figure for the last five, built in 1944. The first 20 from 1941, including the still-running No. 4014, weighed about 540 tonnes (1,189,500 lb). Union Pacific rounds both to 1.2 million pounds.
| 1944 batch, Nos. 4020 to 4024772,250 lb engine plus 436,500 lb loaded tender | 548 t1,210,000 lb |
|---|---|
| 1941 batch, Nos. 4000 to 4019Including No. 4014: 762,000 lb engine plus 427,500 lb loaded tender | 540 t1,190,000 lb |
The American Locomotive Company (ALCO) in Schenectady, New York, built 25 Big Boys for Union Pacific in two orders. The first 20, numbered 4000 to 4019, arrived in 1941. Their engine weighed 762,000 lb (345.6 t) in working order and the loaded tender 427,500 lb (193.9 t), for a total of 1,189,500 lb (539.5 t).
The last five, numbers 4020 to 4024, came in 1944. Wartime restrictions meant heavier steel, and the tender held 1,000 more gallons of water. Those engines weighed 772,250 lb (350.3 t) and their tenders 436,500 lb (198 t), for 1,208,750 lb (548.3 t) in all. That is the 548 tonnes at the top of this page. Union Pacific’s own fact sheet simply rounds both versions to 1.2 million pounds.
So No. 4014, the Big Boy that still runs, is one of the lighter 1941 engines at about 540 tonnes. Union Pacific’s spec sheet for it, revised in September 2024, still lists the original 1941 weights. Wikipedia’s article on No. 4014 gives a higher total of 1,218,500 lb, which Union Pacific does not publish.
A lot of it. The 14-wheel tender on the first order carried 24,000 US gallons (about 90,850 litres) of water and, as built, 28 tons of coal. Water weighs about 8.34 lb per US gallon, so 24,000 gal × 8.34 lb is roughly 200,000 lb (91 t). Add 56,000 lb (25 t) of coal and you get about 256,000 lb, or around 60% of the loaded tender’s 427,500 lb.
That leaves an empty tender of roughly 171,500 lb (78 t), which is our own subtraction rather than an official figure. The National Railroad Museum gives the coal capacity as 32 tons, and Union Pacific’s sheet for No. 4014 lists 25,000 gallons of water. No. 4014 now burns No. 5 fuel oil instead of coal and carries about 6,500 US gallons of it.
The Big Boys were built to haul heavy wartime freight over the Wasatch Mountains between Ogden, Utah, and Cheyenne, Wyoming. According to the National Railroad Museum, one full tender was enough to pull a 3,600-ton train (about 3,270 t) the 55 mountainous miles from Ogden to Echo, Utah.
Pulling power depends on how much weight presses down on the driving wheels. A 1944 Big Boy put 545,200 lb (247 t) on its 16 drivers, and both batches reach a maximum tractive effort of 135,375 lbf (602 kN). To get that much weight around curves, the frame is articulated, or hinged, with two sets of eight driving wheels in a 4-8-8-4 arrangement. With the tender, a Big Boy is about 133 ft (40.5 m) long.
No. 4014 was delivered in December 1941 and retired in December 1961 after running 1,031,205 miles. It then sat on display in Pomona, California, until Union Pacific bought it back in 2013, moved it to Cheyenne and restored it. It returned to service in 2019 for the 150th anniversary of the first transcontinental railroad.
It is the world’s largest operating steam locomotive. In 2026 it made its first coast-to-coast tour for America’s 250th anniversary, running on Norfolk Southern track in the East. It left Cheyenne on March 29, was on display in Sacramento on April 10 and 11 and in Philadelphia on July 4 and 5, and got back to Cheyenne on July 29 after about 7,145 miles.
With a loaded tender, the 1944 Big Boys weighed 1,208,750 lb and the 1941 ones 1,189,500 lb. Union Pacific rounds both to 1.2 million pounds.
Union Pacific’s spec sheet gives No. 4014’s engine as 762,000 lb (345.6 t) and its loaded tender as 427,500 lb (193.9 t), for about 1,189,500 lb (539.5 t) in total. It belongs to the first, slightly lighter batch built in 1941.
It is one of the two usual contenders, the other being the Chesapeake and Ohio H-8 Allegheny. C&O’s engine profiles list the earliest Alleghenies at 771,300 lb, about 950 lb below the 1944 Big Boy’s 772,250 lb, though some claims go as high as 778,000 lb.
Eight of the 25 survive. No. 4014 is the only one that runs; the other seven are on display in St. Louis, Dallas, Omaha, Denver, Scranton, Green Bay and Cheyenne.
About 133 ft (40.5 m) including the tender. Union Pacific’s spec sheet gives 132 ft 9⅞ in for engine and tender together, and Wikipedia gives the engine alone as about 85 ft 3 in (26 m).
Last checked October 3, 2026.
