|Union Pacific Big Boy|
|Big Boy 4014 on display in Pomona, California|
|Builder||American Locomotive Company|
|Build date||1941 (20), 1944 (5)|
|UIC classification||(2′D)D2′ h4|
|Gauge||4 ft 8 1⁄2 in (1,435 mm)|
|36 in (914 mm)|
|Driver diameter||68 in (1,727 mm)|
|42 in (1,067 mm)|
|Wheelbase||72 ft 5.5 in (22.09 m)|
|Length||Locomotive: 85 ft 3.4 in (25.99 m)
Overall: 132 ft 9 1⁄4 in (40.47 m)
|Width||11 ft (3.4 m)|
|Height||16 ft 2 1⁄2 in (4.94 m)|
|Weight on drivers||540,000 lb (244,939.9 kilograms)|
|Locomotive weight||762,000 lb (345,637.4 kilograms)|
|Tender weight||342,200 lb (155,219.3 kilograms) (2/3 load)|
|Locomotive & tender
|1,250,000 lb (566,990.5 kilograms)|
|Fuel capacity||28 short tons (25.4 t; 25.0 long tons)|
|Water capacity||25,000 US gal (95,000 l; 21,000 imp gal)|
|Boiler||95 in (2,400 mm)|
|Boiler pressure||300 lbf/in² (2.1 MPa)|
|Firegrate area||150 sq ft (14 m2)|
Tubes and flues
|5,035 sq ft (468 m2)|
|720 sq ft (67 m2)|
|5,735 sq ft (533 m2)|
|Superheater type||Type A|
|Superheater area||2,043 sq ft (190 m2)|
|Cylinder size||23.75 in × 32 in (603 mm × 813 mm)|
|Top speed||80 mph (130 km/h)|
|Tractive effort||135,375 lbf (602.18 kN)|
|Career||Union Pacific Railroad|
|Last run||July 21, 1959|
|Preserved||4004, 4005, 4006, 4012, 4014, 4017, 4018, 4023|
|Disposition||Eight preserved, remainder scrapped.|
Big Boy was the name of the Union Pacific Railroad's 4000-class 4-8-8-4 articulated steam locomotives, built between 1941 and 1944 by American Locomotive Company (Alco). The 25 Big Boys were the only locomotives to have the 4-8-8-4 wheel arrangement, with two sets of eight driving wheels, a four-wheel leading truck for stability entering curves and a four-wheel trailing truck to support the large firebox.
Union Pacific Railroad (UP) introduced the Challenger-type (4-6-6-4) locomotives in 1936 on its main line across Wyoming. For most of the way the maximum grade is 0.82% in either direction, but the climb eastward from Ogden, Utah into the Wasatch Range (Wahsatch, on the railroad) reached 1.14%. Hauling a 3,600-short-ton (3,300 t; 3,200-long-ton) freight train demanded doubleheading and helper operations, and adding and removing the helper engines from a train slowed operations.
The answer was to design a new locomotive, but for such locomotives to be worthwhile they had to be faster and more powerful than slow mountain luggers like the earlier compound 2-8-8-0s that UP tried after WWI. To avoid locomotive changes, the new class would need to pull long trains at sustained speed—60 miles per hour (100 km/h)—once past the mountain grades. (The 1950s Wyoming Division timetables allowed them to travel at 50 mph (80 km/h) or less, passenger or freight.)
Led by Otto Jabelmann, the UP's design team worked with Alco to re-examine the Challengers, which had been designed by A.H. Fetters. They found that the goals could be achieved by making several changes to the Challenger design, including enlarging the firebox to about 235 by 96 inches (5.97 m × 2.44 m) (about 155 sq ft or 14.4 m2), lengthening the boiler, adding four driving wheels and reducing the size of the driving wheels from 69 to 68 in (1,753 to 1,727 mm).
The Big Boys are articulated, like the Mallet locomotive design. They were designed for stability at 60 miles per hour (100 km/h). They were built with a wide margin of reliability and safety, as they normally operated well below that speed in freight service. Peak horsepower was reached at about 35 mph (56 km/h); optimal tractive effort, at about 10 mph (16 km/h).
Without the tender, the Big Boy had the longest engine body of any reciprocating steam locomotive.
Twenty-five Big Boys were built, in two groups of ten and one of five. All burned coal, with large grates to burn the low-quality Wyoming coal from mines owned by the railroad.
As an experiment Locomotive 4005 was converted to burn oil, but unlike a similar effort with the Challenger types, this was not successful, and the locomotive was soon changed back to coal. The cited reason for this failure was the use of a single burner, which created uneven heating in the Big Boy's large firebox. It is unknown why multiple burners were not employed, though with dieselization in full swing after 1945 the company probably lost interest in steam.
Postwar increases in the price of both coal and labor and the efficiency of diesel-electric motive power foretold a limited life for the Big Boys, but they were among the last steam locomotives taken out of service. Towards the end of the 4000s' career (in the late 1950s) it was found that they could still pull more than their rated tonnage of 3,600 tons (3,300 t). Their ratings were increased several times until they regularly pulled 4,450 short tons (4,040 t) up the Wasatch grade.
The last revenue train hauled by a Big Boy ended its run early in the morning on July 21, 1959. Most were stored operational until 1961, and four remained in operational condition at Green River, Wyoming until 1962. Their duties were assumed by Diesels and gas turbine-electric locomotives) (GTELs.
The Big Boy is well represented among preserved steam locomotives in the United States. All except 4005 and 4017 are in the open without protection from the elements. The dry air of Southern California has helped 4014 to remain well preserved, assisted by care of the local chapter of the Railway and Locomotive Historical Society. The Steamtown example is also said to be in good condition, though the harsher weather of the northeast has taken its toll. The Forney Transportation Museum in Denver moved 4005 to a renovated building in January 2001. Thanks to considerable fundraising and volunteer efforts, 4017 now resides with other pieces of railroad equipment in a climate-controlled shed at the museum in Green Bay. Number 4023 is the only known Big Boy to move by highway since preservation, to the new Kenefick Park in Omaha. Number 4018 is planned to be moved to a new location north of Dallas in Frisco, Texas. There are no operable Big Boys, though Union Pacific announced in late 2012 that it is interested in obtaining a Big Boy and restoring it to operating condition. 4014 is the locomotive the railroad would like to acquire for restoration. It has not yet been decided if 4014 will be the chosen candidate for restoration. If it is restored the engine will join UP's never retired 4-8-4 844 as well as 4-6-6-4 Challenger 3985 in excursion service.
- Eight of the 25 Union Pacific Big Boys still exist:
- 4004: Holliday Park, Cheyenne, Wyoming )
- 4005: Forney Transportation Museum, Denver, Colorado )
- 4006: Museum of Transportation, St. Louis, Missouri )
- 4012: Steamtown National Historic Site, Scranton, Pennsylvania )
- 4014: The Railway and Locomotive Historical Society, Southern California Chapter, Fairplex, Pomona, California )
- 4017: National Railroad Museum, Green Bay, Wisconsin )
- 4018: Museum of the American Railroad, Dallas, Texas )
- 4023: Kenefick Park, Omaha, Nebraska )
Current and past scale model manufactures 
- Rivarossi HO and N scale
- Märklin and Trix HO scale
- Athearn HO & N scale
- Bowser Manufacturing HO scale
- American Z Line Z scale
- Lionel O Scale
- USA Trains 1:29 G Scale
- MTH HO & 1 Scale
Popular culture 
The popular RailSimulator.com franchise Train Simulator Railworks launched a Union Pacific Big Boy add-on. Featured in its famous UP black livery with accurate locomotive simulation, detailed cab interior. The add-on was deemed a success, briefly reaching number 1 on the Steam (software) retail software platform.
- C.B. Peck, ed. (1950). 1950-52 Locomotive Cyclopedia of American Practice. New York: Simmons-Boardman. pp. 501, 519, 523, 545.
- "Union Pacific looking to restore Big Boy for excursion service". Trains. Retrieved 2012-12-08.
Kratville, William W. (1972). Big Boy. Omaha: Kratville Publications.