Union Pacific Big Boy

UP 4000, the first of the class

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

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

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]

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 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).[13][14]

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.[15] Unlike a similar effort with the Challengers, the conversion failed due to uneven heating in the Big Boy's large, single-burner firebox.[15] The locomotive was converted back to coal in 1948.[16][15] (Decades later, No. 4014 would be successfully converted to oil during its restoration.[17]) 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.[18] 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.[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.

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.[21]

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.[11][22]

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.[23]

In 2019, Union Pacific completed the restoration of No. 4014 and placed it in excursion service.[24][25] The locomotive was sent on a tour in celebration of the 150th anniversary of the completion of the First transcontinental railroad.[26][27]

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.

• All tonnage ratings are in exclusion of the engine and tender.
• Tonnage rating from Cheyenne to Buford may be increased by 66.6% with the addition of a helper locomotive.
• Tonnage ratings may be reduced by 10% for fast freight.
• Big Boys saw limited use on the line between Cheyenne and North Platte (in part because they were too long for the turntable in N Platte).

• Drury, George (2015). Guide to North American Steam Locomotives (2nd ed.). Kalmbach Media. ISBN 978-1-62700-259-2.
• Peck, C. B.; Combes, C. L.; et al., eds. (1950). 1950-52 Locomotive Cyclopedia of American Practice (Fourteenth ed.). New York: Simmons-Boardman.
• Solomon, Brian (2009). Alco Locomotives (1st ed.). Voyageur Press. ISBN 978-0-7603-3338-9.

• 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.