Union Pacific 4014 sits in Pine Bluffs, Wyoming during a servicing stop on July 3, 2023.

(Fan Railer, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons)



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 United States. One of them, No. 4014, was re-acquired by Union Pacific, and between 2014 and 2019 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.


The 4-8-8-4 Wheel Arrangement. Front of locomotive on left.

(Gwernol, Public domain, via Wikimedia Commons)




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. 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%. Hauling a 3,600-ton (3,300 t; 3,200-long-ton) freight train demanded double heading and helper operations, which slowed service. So Union Pacific decided to design a new locomotive that could handle the run by itself, faster and more powerful than the compound 2-8-8-0s that UP tried after World War I, and able to pull long trains at a sustained speed of 60 miles per hour (100 km/h) once past mountain grades.


UP 4000, the first of the class, 1941.

(Association of American Railroads, photo by Union Pacific Railroad., Public domain, via Wikimedia Commons)


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. 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). 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. It was designed to travel smoothly and safely at 80 miles per hour.


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

(Sean Lamb (en:User:Slambo), CC BY-SA 2.0 <https://creativecommons.org/licenses/by-sa/2.0>, via Wikimedia Commons)


To achieve these new engineering goals, the 4-6-6-4 Challenger locomotive was "comprehensively redesigned from first principles," wrote locomotive historian Tom Morrison. 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.


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

(Acorns Resort from Milford, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons)


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.

The Big Boys were articulated, like the Mallet locomotive design, although lacking the compounding of the Mallet. 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). The maximum drawbar pull measured during 1943 tests was 138,200 lbf (615,000 N) while starting a train.

The Big Boy has the longest engine body of any reciprocating steam locomotive, longer than two 40-foot buses. 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. There was some speculation that the first series of Chesapeake and Ohio 2-6-6-6 H-8 “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 H-8s, under close scrutiny by the builder and the railroad, found them to be about half a ton less than 772,250 lb (350,290 kg).

A few experiments were carried out on the Big Boys during their years in revenue service. One experiment saw the conversion of No. 4005 to oil fuel in 1946. Unlike a similar effort with the Challengers, the conversion failed due to uneven heating in the Big Boy's large, single-burner firebox. The locomotive was converted back to coal in 1948. (Decades later, No. 4014 would be successfully converted to oil during its restoration.) 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. 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.



The American Locomotive Company manufactured 25 Big Boy locomotives for Union Pacific: 20 in 1941 and five in 1944. 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

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. No. 4007 was modified with a single stack and tested in October 1948. Results were unsatisfactory and locomotive reverted to double stack following tests. No. 4019 given experimental smoke deflectors from 1944 to 1945. No. 4014 in excursion service since May 2019.
4884-2 5 72777–72781 1944 4020-4024


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 that could supply slightly over 12+1⁄2 short tons (25,000 lb) per hour. Water to the boiler was furnished by a Nathan type 4000C Automatic Restarting injector rated for 12,500 gallons per hour on the right side and an Elesco T.P. 502 exhaust steam injector rated for 14,050 gallons per hour on the left side.

Upon their arrival on Union Pacific property in 1941, the Big Boys were assigned to the Utah Division's First Sub, between Ogden and Green River, which included the 1.14% grade for which they were designed. From February 1943 to November of the same year, three Big Boys were assigned to the Los Angeles & Salt Lake Line and ran between Ogden and Milford, Utah. In 1944, with the arrival of additional Challengers and the second order of Big Boys, their operating territory was expanded east from Green River to Cheyenne over the Wyoming Division's Fifth, Sixth, and Seventh Subs. Beginning in 1948, with Challenger locomotives taking over the bulk of service between Ogden and Green River, the Big Boys saw only occasional service on the Utah Division, while their operating territory was expanded to include the line south from Cheyenne to Denver. Between 1950 and 1957, they were occasionally assigned to handle trains east of Cheyenne to North Platte over the Nebraska Division's Third Sub. In the final years of steam on the UP, where the locomotives were only fired up to help with the fall rush traffic, the Big Boys saw service only between Cheyenne and Laramie.

In April 1943, UP borrowed a dynamometer car from AT&SF to evaluate Big Boy performance. Several test runs were made on the Evanston Subdivision from Ogden to Evanston (76 miles), and it was found that a Big Boy could consume 11 tons of coal and 12,000 gallons of water an hour operating at full throttle, producing 6,290 drawbar horsepower at 41.4 mph. Designed to haul 3,600 tons up the 1.14% ruling gradient over this subdivision, the tests demonstrated that a Big Boy could handle 4,200 tons, running at an average speed of 18 to 20 mph between those two division points.

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 (see section on tonnage ratings). 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.

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


Tonnage ratings over principal operating territories (1946-1959)

Union Pacific's Overland Route over which the Big Boys operated had grades that were, by and large, no steeper than 0.82%. Thus for a majority of the route, Big Boys could handle trains in the neighborhood of 6,000 tons in either direction, limited only by the length of sidings and the locomotives' capacity to recharge the train's air brake system. The only two major bottlenecks along the route were the aforementioned 1.14% eastbound gradient between Ogden and Evanston, and the 1.55% westbound gradient between Cheyenne and Sherman Hill; the latter of which was alleviated by the opening of Track 3 via Harriman in 1953, which had a reduced gradient of 0.82% and allowed a Big Boy to theoretically haul a 6,000-ton train unassisted the entire 993 mi (1,598 km) miles from Council Bluffs, Iowa, to Ogden.

To see a list of tonnage ratings, click HERE.


No. 4005 accident

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. Sadly, 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.


 Union Pacific 4-8-8-4 "Big Boy" No. 4006, preserved by the Museum of Transportation, St. Louis, Missouri.

(RFM57, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons)



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.


Surviving Big Boy locomotives

Type Number Date built Serial number Location Notes
4884-1 4004 September 1941 69575 Holliday Park, Cheyenne, Wyoming Received a cosmetic restoration in 2018. Surviving Tender No. 25-C-103.
4884-1 4005 September 1941 69576 Forney Transportation Museum, Denver, Colorado Wrecked on April 27, 1953 and repaired afterwards. Donated to the museum in June 1970.
4884-1 4006 September 1941 69577 National Museum of Transportation, St. Louis, Missouri To receive a cosmetic restoration. Traveled 1,064,625 miles in freight operation, farther than any other Big Boy. Surviving Tender No. 25-C-104.
4884-1 4012 November 1941 69583 Steamtown National Historic Site, Scranton, Pennsylvania Was displayed at Steamtown, USA in Bellows Falls, Vermont, until 1984. Received cosmetic restoration, completed in 2021. Displayed outdoors because it is too large for Steamtown's turntable and roundhouse. Steamtown staff believe No. 4012 could be restored to working order, but recommended first determining whether surrounding rail infrastructure could handle the engine's weight. Surviving Tender No. 25-C-114.
4884-1 4014 November 1941 69585 Union Pacific Railroad, Cheyenne, Wyoming Long displayed at Fairplex RailGiants Train Museum in Pomona, California, 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. Surviving Tender No. 25-C-116. Currently mated with Tender No. 25-C-311 (taken from UP Challenger No. 3985).
4884-1 4017 December 1941 69588 National Railroad Museum, Green Bay, Wisconsin Displayed in a climate-controlled shed. Surviving Tender No. 25-C-404.
4884-1 4018 December 1941 69589 Museum of the American Railroad, Frisco, Texas Moved to its current location from the museum's former location in Dallas, Texas, by rail on August 25, 2013. Surviving Tender No. 25-C-101.
4884-2 4023 November 1944 72780 Kenefick Park, Omaha, Nebraska The only surviving Big Boy from the second group built in 1944, and the only Big Boy known to have been moved by highway. Surviving Tender No. 25-C-105.

UP 4012 on display at Steamtown NHS after its cosmetic restoration which was completed in 2021.

(Fan Railer, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons)


4-8-8-4 Overview

Type and origin
Power type: Steam
Designer: Otto Jabelmann
Builder: American Locomotive Company (ALCO)
Build date: 1941 and 1944
Total produced: 25
​• Whyte 4-8-8-4
• UIC (2′D)D2′ h4g
Gauge: 4 ft 8+1⁄2 in (1,435 mm) standard gauge
Leading diameter: 36 in (914 mm)
Driver diameter: 68 in (1,727 mm)
Trailing diameter: 42 in (1,067 mm)
Minimum curve: 288 ft (88 m) radius/ 20°
Wheelbase, Locomotive: 72 ft 5+1⁄2 in (22.09 m)
Overall: 117 ft 7 in (35.84 m)
Length Locomotive: 85 ft 3+2⁄5 in (25.99 m)
Overall: 132 ft 9+1⁄4 in (40.47 m)
Width: 11 ft (3.35 m)
Height: 16 ft 2+1⁄2 in (4.94 m)
Axle load: 4884-1: 67,500 lb; 4884-2: 68,150 lb
Adhesive weight: 4884-1: 540,000 lb; 4884-2: 545,200 lb
Locomotive weight: 4884-1: 762,000 lb; 4884-2: 772,250 lb
Tender weight: 4884-1: 427,500 lb; 4884-2: 436,500 lb
Total weight: 4884-1: 1,189,500 lb; 4884-2: 1,208,750 lb
Fuel type: Coal (No. 4014 converted to No. 5 fuel oil)
Fuel capacity: 28 tons (25.4 t; 25.0 long tons)
Water capacity. 4884-1: 24,000 US gal; 4884-2: 25,000 US gal
Fuel consumption: Up to 11 tons of coal / hr
Fuel consumption: Up to 12,000 US gal of water / hr
• Firegrate area: 150 sq ft (14 m2)
Boiler: 107 in (2,718 mm) (OD)
Boiler pressure: 300 lbf/in2 (2.1 MPa)
Feedwater heater: Elesco Type T.P. 502 Exhaust Steam Injector, 14,000 US gal/hr capacity
Heating surface: 5,889 sq ft (547 m2) (4884-1); 5,735 sq ft (533 m2) (4884-2)
• Tubes: 967 sq ft (90 m2) (4884-1); 2,734 sq ft (254 m2) (4884-2)
• Flues: 4,218 sq ft (392 m2) (4884-1); 2,301 sq ft (214 m2) (4884-2)
• Tubes and flues: 5,185 sq ft (482 m2) (4884-1); 5,035 sq ft (468 m2) (4884-2)
• Firebox: 704 sq ft (65 m2) (4884-1); 720 sq ft (67 m2) (4884-2)
​• Type: Type E (4884-1), Type A (4884-2)
• Heating area: 2,466 sq ft (229 m2) (Type E);  2,043 sq ft (190 m2) (Type A)
Cylinders: 4
Cylinder size: 23.75 in × 32 in (603 mm × 813 mm)
Valve gear: Walschaerts
Valve type: Piston valves
Valve travel: 7 in (178 mm)
Valve lap: 1+3⁄8 in (35 mm)
Valve lead: 1⁄4 in (6 mm)
Train heating: Steam heat
Locomotive brake: Pneumatic, Schedule 8-ET
Train brakes: Pneumatic
Safety systems: Cab signals
Performance figures
Maximum speed: 80 mph (130 km/h)
Power output: 5,500–6,290 hp (4,100–4,690 kW) @ 41 mph (66 km/h) (drawbar)
Tractive effort: 135,375 lbf (602.18 kN)
Factor of adhesion: 3.99 (4884-1); 4.02 (4884-2)
Operator: Union Pacific Railroad
Class: 4884-1, 4884-2
Road Numbers: 4000-4019 (4884-1); 4020-4024 (4884-2)
Last run: June 21, 1959 (revenue service)
Retired: 1959–1962
Preserved: Eight preserved, remainder scrapped
Restored: No. 4014; May 1, 2019
Disposition: Seven on display and one (No. 4014) operational in excursion service
Cost to build: US$ 265,000 in 1941, equivalent to $5,272,421 in 2022


Last of the Giants (1959)

Last of the Giants is an excellent publicity film produced by the Union Pacific Railroad which gives the viewer an in-depth look at their 4-8-8-4 Big Boy locomotives.