Union Pacific Big Boy

The Union Pacific Big Boy is a type of simple articulated 4-8-8-4 steam locomotive manufactured by the American Locomotive Company between 1941 and 1944 and operated by the Union Pacific Railroad in revenue service until 1959.

Union Pacific Big Boy
Union Pacific 4014 running through Friesland, Wisconsin, on July 25, 2019
Type and origin
Power typeSteam
BuilderAmerican Locomotive Company
Build date1941–1944
Total produced25
  UIC(2′D)D2′ h4
Gauge4 ft 8 12 in (1,435 mm) standard gauge
Leading dia.36 in (914 mm)
Driver dia.68 in (1,727 mm)
Trailing dia.42 in (1,067 mm)
Wheelbase72 ft 5.5 in (22.09 m)
LengthLocomotive: 85 ft 3.4 in (25.99 m)
Overall: 132 ft 9 14 in (40.47 m)
Width11 ft (3.4 m)
Height16 ft 2 12 in (4.94 m)
Axle load67,800 lb (30,800 kg)
Adhesive weight540,000 lb (245,000 kg)
Loco weight762,000 lb (345,600 kg) (381t)
Tender weight342,200 lb (155,220 kg) (2/3 load)
Total weight1,250,000 lb (567,000 kg)
Fuel typeCoal; No. 4014 converted to No 5 fuel oil
Fuel capacity28 short tons (25.4 t; 25.0 long tons)
Water cap4884-1: 24,000 US gal (91,000 l; 20,000 imp gal)
4884-2: 25,000 US gal (95,000 l; 21,000 imp gal)
  Firegrate area
150 sq ft (14 m2)
Boiler95 in (2,400 mm)
Boiler pressure300 lbf/in2 (2.1 MPa)
Heating surface5,735 sq ft (533 m2)
  Tubes and flues5,035 sq ft (468 m2)
  Firebox720 sq ft (67 m2)
  TypeType E (Nos. 4000-4019), Type A (Nos. 4020-4024)
  Heating area2,043 sq ft (190 m2)
Cylinder size23.75 in × 32 in (603 mm × 813 mm)
Performance figures
Maximum speed80 mph (130 km/h)
Power output5,300–7,000 hp (4,000–5,200 kW) @ 41 mph
Tractive effort135,375 lbf (602.18 kN)
Factor of adh.3.99
OperatorsUnion Pacific Railroad
Class4884-1, 4884-2
Last runJune 21, 1959 (Revenue)
Preserved4004, 4005, 4006, 4012, 4014, 4017, 4018, 4023
DispositionEight preserved (No. 4014 operational), remainder scrapped
Cost to build US$ 265,000 in 1941, equivalent to $4,513,993 in 2018

The 25 Big Boy locomotives were built to haul freight over the Wasatch mountains between Ogden, Utah, and Green River, Wyoming. In the late 1940s, they were reassigned to Cheyenne, Wyoming, where they hauled freight over Sherman Hill to Laramie, Wyoming. They were the only locomotives to use a 4-8-8-4 wheel arrangement: four-wheel leading truck for stability entering curves, two sets of eight driving wheels and a four-wheel trailing truck to support the large firebox.

Eight Big Boys survive, most on static display at museums across the country. One of them, No. 4014, was re-acquired by Union Pacific and restored to operating condition in 2019, regaining the title as the largest and most powerful operating steam locomotive in the world.



In 1936, Union Pacific introduced the Challenger-type (4-6-6-4) locomotives on its main line over the Wasatch Range between Green River and Ogden.[2][3] For most of the route, the maximum grade is 0.82% in either direction, but the climb eastward from Ogden, into the Wasatch Range, reached 1.14%.[4] Hauling a 3,600-short-ton (3,300 t; 3,200-long-ton) freight train demanded double heading and helper operations, which slowed service.[3][4] So Union Pacific decided to design a new locomotive that could handle the run by itself:[5] faster and more powerful than the compound 2-8-8-0s that UP tried after World War I, able to pull long trains at a sustained speed of 60 miles per hour (100 km/h) once past mountain grades.[5]

Led by Otto Jabelmann, the head of the Research and Mechanical Standards section of the UPRR (Union Pacific Railroad) Mechanical Department, the UP design team worked with ALCO (the American Locomotive Company) to re-examine their Challenger locomotives.[6] The team found that Union Pacific's goals could be achieved by enlarging its firebox to about 235 by 96 inches (5.97 m × 2.44 m) (about 150 sq ft or 14 m2), increasing boiler pressure to 300 psi, adding four driving wheels, and reducing the size of the driving wheels from 69 to 68 in (1,753 to 1,727 mm) on a new engine.[6] The new locomotive was carefully designed not to exceed an axle loading of 67,800 lb, and achieved the maximum possible starting tractive effort with a factor of adhesion of 4.0.[4][6] It was designed to travel smoothly and safely at 80 miles per hour (130 km/h), even though it was not intended to be used that fast.[7]

To achieve these new engineering goals, the locomotive was "comprehensively redesigned from first principles," wrote locomotive historian Tom Morrison.[8] The overall design simplified some aspects of previous locomotive designs and added complexity elsewhere. Compounding, booster, and feed water heaters were eliminated, as were Baker valves and limited cut-off. But the "proliferation of valves and gauges on the backhead showed that running a Big Boy was an altogether more complicated and demanding task for the crew than running previous existing locomotives," Morrison wrote.[8]

The 4-8-8-4 class series, originally rumored to be called the "Wasatch", acquired its nickname after an unknown worker scrawled "Big Boy" in chalk on the front of No. 4000, then under construction as the first of its class.[4][5][9]

The Big Boys were articulated, like the Mallet locomotive design, though lacking the compounding of the Mallet.[10] They were built with a wide margin of reliability and safety, and normally operated well below 60 miles per hour (100 km/h) in freight service. Peak horsepower was reached at about 41 mph (66 km/h).[11] The maximum drawbar pull measured during 1943 tests was 138,000 lbs while starting a train.[11]

The Big Boy has the longest engine body of any reciprocating steam locomotive, longer than two buses.[12] It was likely the heaviest steam locomotive ever built: the 772,250-lb (350,286-kg)engine and 436,500-lb (197,993-kg) tender together outweighed a Boeing 747.[12] There is some speculation that the first series of Chesapeake and Ohio 2-6-6-6 “Allegheny” locomotives, built by the Lima Locomotive Works in 1941, may have weighed as much as 778,200 lbs, exceeding the Big Boys, but subsequent re-weighs of early-production H8s, under close scrutiny by the builder and the railroad, found them to be less than 772,250 lbs.[13][14]


The American Locomotive Company manufactured 25 Big Boy locomotives for Union Pacific; a group of 20 in 1941 and one group of five in 1944.[4][9] Along with the Challengers, the Big Boys arrived on the scene just as traffic was surging in preparation for American participation in World War II.

Table of orders and numbers[15]
Class Quantity Serial Nos. Year built UP No. Notes
4884-1 20 69571-69590 1941 4000-4019 No. 4005 converted to oil fuel in 1946 and reverted to coal in 1948.[16] No. 4019 given experimental smoke deflectors in 1944-45, later removed.[17] No. 4014 in excursion service since May 2019.[18] Nos. 4004, 4005, 4006, 4012, 4017 and 4018 on display in various locations.[16]
4884-2 5 72777–72781 1944 4020-4024 No. 4023 on display in Omaha, Nebraska.[16]


The Big Boy locomotives had large grates to burn the low-quality bituminous coal supplied by Union Pacific-owned mines in Wyoming. Coal was carried from the tender to the firebox by a stoker motor: a steam engine that drove an Archimedes’ screw.

As an experiment, No. 4005 was converted to burn oil.[19] Unlike a similar effort with the Challengers, the conversion failed due to uneven heating in the Big Boy's large, single-burner firebox.[19] The locomotive was converted back to coal firing in 1948.[16][19] By contrast, No. 4014 was successfully converted to oil during its restoration.[18] Another short-term experiment was the fitting of smoke deflectors on locomotive 4019, similar to those found on the railroad’s FEF Series. These were later removed, as the Big Boys' nozzle and blower in the smoke box could blow smoke high enough to keep engineers’ lines of sight clear.

The locomotives were held in high regard by crews, who found them sure-footed and more “user friendly” than other motive power. They were capable machines; their rated hauling tonnage was increased several times over the years. But postwar increases in the price of coal and labor, along with the advent of efficient, cost-effective diesel-electric power, spelled the end of their operational lives. Nonetheless, they were among the last steam locomotives withdrawn from service on the Union Pacific. 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 diesel locomotives and gas turbine-electric locomotives.[20]

In 2019, Union Pacific completed the restoration of No. 4014, and placed it in excursion service.[21][22] The locomotive was sent on a tour in celebration of the 150th anniversary of the completion of the First Transcontinental Railroad.[23][24]

Accidents and incidents

  • On April 27, 1953, No. 4005 was pulling a freight train through southern Wyoming when it jumped a switch track at 50 mph (80 km/h), throwing the engine onto its left side and derailing its tender and the first 18 freight cars of its 62-car train. The engineer and fireman were killed on impact; the brakeman died of severe burns in a hospital a few days later. The tender destroyed the cab of the locomotive, and the loads from the 18 derailed cars were scattered. Despite the relatively heavy damage, the locomotive was repaired by Union Pacific at its Cheyenne facility and returned to service.[25]
  • On May 16, 2019, No. 4014 partially derailed, with two axles from one set of drive wheels falling into the gauge, while entering the yard at Rawlins, Wyoming;[26] the derailed drive wheels were returned to the rails within three hours.[27]


Of the original 25 Big Boy locomotives, seven are on static display: two of these are displayed indoors while the other five are displayed outdoors without protection from the elements. An eighth, Union Pacific 4014, has been restored to operating condition by Union Pacific as part of its steam program.[16]

Surviving Big Boy locomotives[15][16]
Type Number Image Date built Serial number Location Coordinates Notes
4884-1 4004 September 1941 69575 Holliday Park, Cheyenne, Wyoming 41°08′12.30″N 104°47′59.4″W Received a cosmetic restoration in 2018.[28]
4884-1 4005 September 1941 69576 Forney Transportation Museum, Denver, Colorado 39°46′37.38″N 104°58′13.8″W Donated to the museum in June 1970.[19][29]
4884-1 4006 September 1941 69577 Museum of Transportation, St. Louis, Missouri 38°34′19.73″N 090°27′40.0″W Traveled 1,064,625 miles in freight operation, farther than any other Big Boy.[16]
4884-1 4012 November 1941 69583 Steamtown National Historic Site, Scranton, Pennsylvania 41°24′26.96″N 075°40′10.8″W Was displayed at Steamtown, USA in Bellows Falls, Vermont, until 1984. Currently undergoing cosmetic restoration.[30] Displayed outdoors[31] because it is too large for Steamtown's turntable and roundhouse.[31] Steamtown staff believe No. 4012 could be restored to working order, but recommended first determining whether surrounding "track, switches, culverts, trestles, bridges, wyes, turntables and other facilities [could] bear her great weight".[31][32]
4884-1 4014 November 1941 69585 Union Pacific Railroad, Cheyenne, Wyoming 41°7′46.9308″N 104°48′49.1688″W Long displayed at Fairplex RailGiants Train Museum in Pomona, California,[33] No. 4014 was re-acquired and restored to operational shape by Union Pacific, then placed in excursion service in May 2019 at its new home in Cheyenne, Wyoming, as the largest, heaviest, and most powerful operational steam locomotive in the world.[18]
4884-1 4017 December 1941 69588 National Railroad Museum, Green Bay, Wisconsin 44°29′02.70″N 088°02′55.1″W Displayed in a climate-controlled shed.[16]
4884-1 4018 December 1941 69589 Museum of the American Railroad, Frisco, Texas 33.144513°N 96.833444°W / 33.144513; -96.833444 (Big Boy 4018) Moved to its current location from the museum's former location in Dallas, Texas, by rail on August 25, 2013.[34]
4884-2 4023 November 1944 72781 Kenefick Park, Omaha, Nebraska 41°13′55.7″N 095°55′4.1″W The only Big Boy known to have been moved by highway.[16]

See also

Notes and references


  1. Peck, Combes & Augur 1950, pp. 501, 519, 523, 545.
  2. "Challenger No. 3985". Union Pacific. Archived from the original on May 1, 2019. Retrieved May 23, 2019.
  3. Solomon 2009, p. 70.
  4. Glischinski, Steve (August 21, 2013). "Big Boy story began in 1940". Trains. Archived from the original on June 24, 2018. Retrieved July 11, 2019.
  5. Welsh, Joe; Boyd, Jim; Howes Jr., William F. (2006). The American Railroad: Working for the Nation (1st ed.). MBI Publishing. p. 104. ISBN 978-0-7603-1631-3.
  6. Vantuono, William C. (July 9, 2019). "Railway Age, October 4, 1941: UP's "Big Boy" debuts". Railway Age. Archived from the original on July 11, 2019. Retrieved July 11, 2019.
  7. Elliott, Dan (April 15, 2014). "Huge Big Boy steam locomotive coming back to life". Yahoo! News. Associated Press. Retrieved April 15, 2014.
  8. Morrison, Tom (2018-07-10). The American Steam Locomotive in the Twentieth Century. McFarland. ISBN 9781476627939.
  9. Solomon 2009, p. 75.
  10. Morrison, Tom (2018). The American Steam Locomotive in the Twentieth Century (1st ed.). McFarland & Company. pp. 533–534. ISBN 978-1-4766-6582-5.
  11. Kratville, William (1972). Big Boy. Kratville Publications.
  12. Gruver, Mead (May 8, 2019). "Refurbished 'Big Boy' locomotive weighs more than a Boeing 747". The Associated Press. USA Today. Archived from the original on June 8, 2019. Retrieved June 12, 2019.
  13. Solomon, Brian (2000). Union Pacific Railroad. Railroad Color History (1st ed.). Voyageur Press. p. 104. ISBN 0-7603-0756-3.
  14. King, Ed (February 15, 2018). "Big Boy versus Allegheny". Trains. Archived from the original on September 28, 2018. Retrieved July 11, 2019.
  15. Drury 2015, p. 319.
  16. Wrinn, Jim (February 15, 2018). "Where to find Big Boy locomotives". Trains. Archived from the original on September 28, 2018. Retrieved June 16, 2019.
  17. "Marklin 37994 Union Pacific "Big Boy" Steam Loco". Trainz.com. Archived from the original on July 11, 2019. Retrieved July 11, 2019.
  18. "Big Boy No. 4014". Union Pacific. Archived from the original on May 17, 2019. Retrieved June 5, 2019.
  19. Frank, Al. "Big Boy". Forney Museum of Transportation. Archived from the original on December 10, 2018. Retrieved July 27, 2019.
  20. Klein, Maury (2006). Union Pacific: Volume II, 1894-1969 (2nd ed.). University of Minnesota Press. p. 479. ISBN 978-0-8166-4460-5.
  21. Scott, Ramsey (May 4, 2019). "The Big Boy leaves the shop and heads into history". Wyoming Tribune Eagle. Archived from the original on May 4, 2019. Retrieved July 30, 2019.
  22. Sweeney, Steve (May 4, 2019). "UP steam crew has Big Boy ready to roll". Trains. Archived from the original on May 6, 2019. Retrieved July 30, 2019.
  23. "2019 Union Pacific Steam Schedule". Union Pacific. Archived from the original on April 30, 2019. Retrieved July 25, 2019.
  24. "World's largest locomotive coming to West Chicago to celebrate 150th anniversary of Transcontinental Railroad completion". WLS-TV. July 24, 2019. Archived from the original on July 25, 2019. Retrieved July 25, 2019.
  25. "DISASTER ON THE RAILS: The Wreck of the 4005". Forney Museum of Transportation. Archived from the original on September 24, 2018. Retrieved June 14, 2019.
  26. Wrinn, Jim (May 16, 2019). "Big Boy stubs its toe with derailment". Trains. Archived from the original on May 22, 2019. Retrieved June 9, 2019.
  27. Cobb, Debbie (May 17, 2019). "Big Boy Derailed On Way To Laramie, Back On Schedule". KCGY. Archived from the original on May 17, 2019. Retrieved June 9, 2019.
  28. "Cheyenne's Big Boy 4004 to shine in new paint July 9". Trains. June 26, 2018. Archived from the original on April 14, 2019. Retrieved July 12, 2019.
  29. King, Kat (September 4, 2009). "The Forney Museum is worth getting around to". The Denver Post. Archived from the original on July 27, 2019. Retrieved July 27, 2019.
  30. https://www.nps.gov/stea/learn/news/steamtown-national-historic-site-s-union-pacific-big-boy-no-4012-removed-from-public-display-for-cosmetic-restoration-and-painting.htm
  31. Chappell, Gordon. "Union Pacific Railroad No. 4012". Steam Over Scranton: Special History Study, American Steam Locomotives. National Park Service. Archived from the original on August 13, 2012. Retrieved March 13, 2012.
  32. "Steamtown's Locomotives and Cars". Steamtown National Historic Site. National Park Service. Retrieved March 13, 2012.
  33. "Union Pacific "Big Boy" 4014". RailGiants Train Museum. Archived from the original on April 27, 2019. Retrieved July 13, 2019.
  34. "Big Boy Steam Locomotive Arrives at its New Home in Frisco". Museum of the American Railroad. August 26, 2013. Archived from the original on April 13, 2016. Retrieved July 28, 2019.


Further reading

  • Bush, John E.; Ehernberger, James L. (1996). Union Pacific Steam Big Boy Portraits (1st ed.). Challenger Press. ASIN B0027ZOGLA.
  • Reisdorff, James J. (2007). The Big Legacy of the Union Pacific Big Boy: Why Railfans Still Love the "World's Largest" Steam Locomotive (1st ed.). South Platte Press. ISBN 978-0942035735.
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