Union Pacific Big Boy

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Union Pacific Big Boy
Union Pacific 4014 Big Boy.jpg
Union Pacific 4014 passes through Keller, Texas, on August 13, 2021
Type and origin
Power typeSteam
DesignerOtto Jabelmann
BuilderAmerican Locomotive Company (ALCO)
Build date1941 and 1944
Total produced25
 • Whyte4-8-8-4
 • 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)
Minimum curve288 ft (88 m) radius/ 20°
  • Locomotive: 72 ft 5+12 in (22.09 m)
  • Overall: 117 ft 7 in (35.84 m)
LengthLocomotive: 85 ft 3+25 in (25.99 m)
Overall: 132 ft 9+14 in (40.47 m)
Width11 ft (3.35 m)
Height16 ft 2+12 in (4.94 m)
Axle load4884-1: 67,500 lb
4884-2: 68,150 lb
Adhesive weight4884-1: 540,000 lb
4884-2: 545,200 lb
Loco weight4884-1: 762,000 lb
4884-2: 772,250 lb
Tender weight4884-1: 427,500 lb
4884-2: 436,500 lb
Total weight4884-1: 1,189,500 lb
4884-2: 1,208,750 lb
Fuel typeCoal (No. 4014 converted to No. 5 fuel oil)
Fuel capacity28 short tons
(25.4 t; 25.0 long tons)
Water cap.4884-1: 24,000 US gal
4884-2: 25,000 US gal
Fuel consumptionUp to 11 short tons of coal / hr
Up to 12,000 US gal of water / hr
 • Firegrate area
150 sq ft (14 m2)
Boiler107 in (2,718 mm) (OD)
Boiler pressure300 lbf/in2 (2.1 MPa)
Feedwater heaterElesco Type T.P. 502 Exhaust Steam Injector
14,000 US gal/hr capacity
Heating surface5,889 sq ft (547 m2) (4884-1)
5,735 sq ft (533 m2) (4884-2)
 • Tubes967 sq ft (90 m2) (4884-1)
2,734 sq ft (254 m2) (4884-2)
 • Flues4,218 sq ft (392 m2) (4884-1)
2,301 sq ft (214 m2) (4884-2)
 • Tubes and flues5,185 sq ft (482 m2) (4884-1)
5,035 sq ft (468 m2) (4884-2)
 • Firebox704 sq ft (65 m2) (4884-1)
720 sq ft (67 m2) (4884-2)
 • TypeType E (4884-1)
Type A (4884-2)
 • Heating area2,466 sq ft (229 m2) (Type E)
2,043 sq ft (190 m2) (Type A)
Cylinder size23.75 in × 32 in
(603 mm × 813 mm)
Valve gearWalschaerts
Valve typePiston valves
Valve travel7 in (178 mm)
Valve lap1+38 in (35 mm)
Valve lead14 in (6 mm)
Train heatingSteam heat
Loco brakePneumatic, Schedule 8-ET
Train brakesPneumatic
Safety systemsCab signals
Performance figures
Maximum speed80 mph (130 km/h)
Power output5,500–6,290 hp (4,100–4,690 kW) @ 41 mph (66 km/h) (drawbar)
Tractive effort135,375 lbf (602.18 kN)
Factor of adh.3.99 (4884-1)
4.02 (4884-2)
OperatorsUnion Pacific Railroad
Class4884-1, 4884-2
Numbers4000-4019 (4884-1)
4020-4024 (4884-2)
Last runJune 21, 1959 (revenue service)
PreservedEight preserved, remainder scrapped
RestoredNo. 4014; May 1, 2019
DispositionSeven on display and one (No. 4014) operational in excursion service
Cost to build US$ 265,000 in 1941, equivalent to $4,882,115 in 2021

The Union Pacific Big Boy is a type of simple articulated 4-8-8-4 steam locomotive manufactured by the American Locomotive Company (ALCO) between 1941 and 1944 and operated by the Union Pacific Railroad in revenue service until 1962.

The 25 Big Boy locomotives were built to haul freight over the Wasatch Range 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.

Today, eight Big Boys survive, with most on static display at museums across the USA. One of them, No. 4014, was re-acquired by Union Pacific, and between 2014 and 2019 it was rebuilt to operating condition for the 150th anniversary of the first transcontinental railroad. It thus regained 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]

A Union Pacific design team led by Otto Jabelmann, the head of the Research and Mechanical Standards section of the Union Pacific's Mechanical Department, worked with ALCO (the American Locomotive Company) to re-examine their Challenger locomotives.[6] The team found that the railroad's goals could be achieved by enlarging the Challenger 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).[6] The new locomotive was carefully designed not to exceed an axle loading of 67,800 lb (30,800 kg), 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.[7]

To achieve these new engineering goals, the Challenger 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 valve gear 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]

UP 4000, the first of the class

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

The Big Boys were articulated, like the Mallet locomotive design, although 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 drawbar horsepower was reached at about 41 mph (66 km/h).[11] The maximum drawbar pull measured during 1943 tests was 138,200 lbf (615,000 N) while starting a train.[11]

The Big Boy has the longest engine body of any reciprocating steam locomotive, longer than two 40-foot buses.[12] They were also the heaviest reciprocating steam locomotives ever built; the combined weight of the 772,250 lb (350,290 kg) engine and 436,500 lb (198,000 kg) tender outweighed a Boeing 747.[12] There was 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 lb (353,000 kg), 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 lb (350,290 kg).[13][14]


The American Locomotive Company manufactured 25 Big Boy locomotives for Union Pacific: 20 in 1941 and 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. 4007 was modified with a single stack and tested in October 1948. Results were unsatisfactory and locomotive reverted to double stack following tests.[17] No. 4019 given experimental smoke deflectors from 1944 to 1945.[18] No. 4014 in excursion service since May 2019.[19]
4884-2 5 72777–72781 1944 4020-4024


The cab controls of No. 4017 at the National Railroad Museum in Green Bay, Wisconsin

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 Standard Stoker Company type MB automatic stoker capable of supplying slightly over 12+12 short tons (25,000 lb) per hour. Water was injected into the boiler by a Nathan type 4000C Automatic Restarting injector (up to 12,500 gallons per hour) on the right side and an Elesco T.P. 502 exhaust steam injector (up to 14,050 gallons per hour) on the left side. A Big Boy could consume 11 tons of coal and 12,000 gallons of water an hour operating at maximum power.[11]

As an experiment, No. 4005 was converted to burn oil in 1946.[20] Unlike a similar effort with the Challengers, the conversion failed due to uneven heating in the Big Boy's large, single-burner firebox.[20] The locomotive was converted back to coal in 1948.[16][20] (Decades later, No. 4014 would be successfully converted to oil during its restoration.[19]) Another experiment saw No. 4007 being modified with a single stack in October 1948. The results were unsatisfactory, and the locomotive was reverted to double stack after testing.[17] One final short-term experiment was the fitting of smoke deflectors on locomotive 4019, similar to those found on the railroad's FEF Series, as well as some of their Challengers. 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.

A close-up of No. 4014's running gear in 2019

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, and 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; four remained in operational condition at Green River, Wyoming until 1962. Their duties were assumed by diesel locomotives and gas turbine-electric locomotives.[21]

In 2019, Union Pacific completed the restoration of No. 4014 and placed it in excursion service.[22][23] The locomotive was sent on a tour in celebration of the 150th anniversary of the completion of the First transcontinental railroad.[24][25]


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. The locomotive was repaired by Union Pacific at its Cheyenne facility and returned to service until 1962.[26]


Most of the 25 Big Boys were scrapped, but seven remain on static display—two indoors and five outdoors, under the elements—and an eighth, Union Pacific 4014, was rebuilt to operating condition by Union Pacific's steam program.[16]

Surviving Big Boy locomotives[15][16]
Type Number Image Date built Serial number Location Coordinates Notes
4884-1 4004 A Big 'un (38031091034).jpg September 1941 69575 Holliday Park, Cheyenne, Wyoming 41°08′12.30″N 104°47′59.4″W / 41.1367500°N 104.799833°W / 41.1367500; -104.799833 (Big Boy 4004) Received a cosmetic restoration in 2018.[27] Surviving Tender No. 25-C-103.
4884-1 4005 Union Pacific "Big Boy" No. 4005 (4 8 8 4) (26512531825).jpg September 1941 69576 Forney Transportation Museum, Denver, Colorado 39°46′37.38″N 104°58′13.8″W / 39.7770500°N 104.970500°W / 39.7770500; -104.970500 (Big Boy 4005) Donated to the museum in June 1970.[20][28]
4884-1 4006 Big Boy 4006.jpg September 1941 69577 National Museum of Transportation, St. Louis, Missouri 38°34′19.73″N 090°27′40.0″W / 38.5721472°N 90.461111°W / 38.5721472; -90.461111 (Big Boy 4006) Traveled 1,064,625 miles in freight operation, farther than any other Big Boy.[16] Surviving Tender No. 25-C-104.
4884-1 4012 Union Pacific Big Boy 4012 May 10th, 2021.jpg November 1941 69583 Steamtown National Historic Site, Scranton, Pennsylvania 41°24′26.96″N 075°40′10.8″W / 41.4074889°N 75.669667°W / 41.4074889; -75.669667 (Big Boy 4012) Was displayed at Steamtown, USA in Bellows Falls, Vermont, until 1984. Received cosmetic restoration, completed in 2021.[29] Displayed outdoors[30] because it is too large for Steamtown's turntable and roundhouse.[30] Steamtown staff believe No. 4012 could be restored to working order, but recommended[when?] first determining whether surrounding rail infrastructure could handle the locomotive's weight.[30][31] Surviving Tender No. 25-C-114.
4884-1 4014 Union Pacific Big Boy 4014 Cajon Summit October 13th, 2019.jpg November 1941 69585 Union Pacific Railroad, Cheyenne, Wyoming 41°7′46.9308″N 104°48′49.1688″W / 41.129703000°N 104.813658000°W / 41.129703000; -104.813658000 (Big Boy 4014) Long displayed at Fairplex RailGiants Train Museum in Pomona, California,[32] No. 4014 was re-acquired and restored to operational condition 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.[19] Surviving Tender No. 25-C-116.[33] Currently mated with Tender No. 25-C-311 (taken from UP Challenger No. 3985).[a]
4884-1 4017 UP Big Boy 4017 front end.jpg December 1941 69588 National Railroad Museum, Green Bay, Wisconsin 44°29′02.70″N 088°02′55.1″W / 44.4840833°N 88.048639°W / 44.4840833; -88.048639 (Big Boy 4017) Displayed in a climate-controlled shed.[16] Surviving Tender No. 25-C-404.[37]
4884-1 4018 Union Pacific Big Boy 4-8-8-4 -4018 at Fair Park 09.2006 0.jpg December 1941 69589 Museum of the American Railroad, Frisco, Texas 33°08′40″N 96°50′00″W / 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.[38] Surviving Tender No. 25-C-101.[39]
4884-2 4023 Big Boy 4023.jpg November 1944 72780 Kenefick Park, Omaha, Nebraska 41°13′55.7″N 095°55′4.1″W / 41.232139°N 95.917806°W / 41.232139; -95.917806 (Big Boy 4023) The only surviving Big Boy from the second group built in 1944, and the only Big Boy known to have been moved by highway. [16] Surviving Tender No. 25-C-105.  

See also[edit]

Notes and references[edit]


  1. ^ In 2019, the tender from Challenger No. 3985, No. 25-C-311 was connected to Big Boy No. 4014 to save time in meeting the restoration deadline.[34][35] The Railroading Heritage of Midwest America (RRHMA), which acquired No. 3985 in 2022, planned to rebuild No. 4014's original tender, No. 25-C-116, to carry fuel oil instead of coal.[36] Afterwards, it will eventually be reconnected with No. 4014, with the No. 25-C-311 tender to be reconnected to the No. 3985 locomotive.[36]


  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. ^ a b Solomon 2009, p. 70.
  4. ^ a b c d e 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. ^ a b c 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. ^ a b c 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. ^ a b Morrison, Tom (2018-07-10). The American Steam Locomotive in the Twentieth Century. McFarland. ISBN 9781476627939.
  9. ^ a b 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. ^ a b c Kratville, William (1972). Big Boy. Kratville Publications.
  12. ^ a b Gruver, Mead (May 8, 2019). "Refurbished 'Big Boy' locomotive weighs more than a fully loaded 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. ^ a b Drury 2015, p. 319.
  16. ^ a b c d e f g 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. ^ a b Report of Tests (PDF) (Technical report). Union Pacific Railroad Company Research & Mechanical Standards.
  18. ^ "Marklin 37994 Union Pacific "Big Boy" Steam Loco". Trainz.com. Archived from the original on July 11, 2019. Retrieved July 11, 2019.
  19. ^ a b c "Big Boy No. 4014". Union Pacific. Archived from the original on May 17, 2019. Retrieved June 5, 2019.
  20. ^ a b c d Frank, Al. "Big Boy". Forney Museum of Transportation. Archived from the original on December 10, 2018. Retrieved July 27, 2019.
  21. ^ Klein, Maury (2006). Union Pacific: Volume II, 1894-1969 (2nd ed.). University of Minnesota Press. p. 479. ISBN 978-0-8166-4460-5.
  22. ^ 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.
  23. ^ 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.
  24. ^ "2019 Union Pacific Steam Schedule". Union Pacific. Archived from the original on April 30, 2019. Retrieved July 25, 2019.
  25. ^ "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.
  26. ^ "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.
  27. ^ "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.
  28. ^ 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.
  29. ^ "Steamtown National Historic Site's Union Pacific "Big Boy" No. 4012 Removed From Public Display For Cosmetic Restoration and Painting - Steamtown National Historic Site (U.S. National Park Service)".
  30. ^ a b c 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.
  31. ^ "Steamtown's Locomotives and Cars". Steamtown National Historic Site. National Park Service. Retrieved March 13, 2012.
  32. ^ "Union Pacific "Big Boy" 4014". RailGiants Train Museum. Archived from the original on April 27, 2019. Retrieved July 13, 2019.
  33. ^ QBL1201713 (2008-06-14), English: Tender Classification Plate 25-C-116 (UP 4014). Original plate number still intact after 72 years., retrieved 2022-11-23
  34. ^ Wrinn, Jim (March 31, 2020). "Union Pacific No. 3985's next stop". Trains. Kalmbach Publishing. Archived from the original on January 17, 2021. Retrieved June 24, 2021.
  35. ^ Wrinn, Jim (2020). Union Pacific's Big Boys: The Complete Story from History to Restoration (1st ed.). Kalmbach Books. p. 143. ISBN 978-1627007924.
  36. ^ a b Kratville-Wrinn, Cate (April 28, 2022). "Original Big Boy tender to be restored by Railroading Heritage of Midwest America". Trains. Kalmbach Publishing. Archived from the original on May 13, 2022. Retrieved May 16, 2022.
  37. ^ "National Railroad Museum - Green Bay, WI 54304". National Railroad Museum. Retrieved 2022-11-20.
  38. ^ "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.
  39. ^ "Museum of the American Railroad". Museum of the American Railroad. Retrieved 2022-11-20.


Further reading[edit]

  • 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.
  • Wrinn, Jim (2020). Union Pacific's Big Boys: The Complete Story from History to Restoration (1st ed.). Kalmbach Media. ISBN 978-1-62700-792-4.

External links[edit]