Alco 4-8-8-4
UP Big Boy

Union Pacific Railroad

Model Description
Now updated to Version 2! From its highly detailed and accurate shape to the custom 4-8-8-4 sounds, this locomotive has been painstakingly recreated and is bound to please even the most ardent MSTS steam fan. Over six months in development with the talented help of the MSTS community, the UP Big Boy features highly detailed and realistic textures, superb sounds, accurate physics, right down to the custom-built cab. Whether you run in a pure steam environment, transition period, or even just run your own excursions, this magnificent recreation of one of steam's most legendary locomotives is certainly worth the download. Available as part of the UP Steam Pack.

The Union Pacific Big Boy, the largest successful steam engine ever built, was borne out of necessity. The 1.55% continuous grade up Sherman Hill in the Wasatch mountain region (just east of Ogden, Utah) required Union Pacific to maintain helper service that saw as many as three extra locomotives being added to freights. Especially problematic was the coal service that originated in the Powder River basin.

To overcome this challenging terrain, Union Pacific's engineering department was assigned the task of designing an engine that could handle a 3600-ton train over Sherman Hill unassisted. The engineering team's design requirements specified a tractive effort of 135,000 lbs and a weight on drivers of 540,000 lbs, assuming an adhesion factor of 4.0. To meet an axle loading requirement of 67,500 lbs, this would require 8 drivers or an x-8-8-x wheel arrangement, and thus the designers agreed upon the 4-8-8-4 design. Horsepower and cylinder sizes were based on 300 psi boiler pressure. The locomotive was designed for a maximum speed of 80 mph, although it was unlikely that this speed would ever be reached. This provided a generous safety factor in the design. Maximum power and tractive effort was seen at around 30 mph. Except for one experiment involving converting #4005 temporarily into an oil burner, these engines were built as coal-burners.

Alco built 25 of these 4-8-8-4s in two groups starting in 1941. The first group (Class 1) were numbered 4000-4019. The second group (Class 2) were numbered 4020-4024. The first Big Boy, number 4000, was delivered to Omaha at 6 p.m., September 5th, 1941.

Big Boys were large! A Big Boy had over one mile of boiler tubes and flues, and the firebox grate measured 150 square feet. The engines had 25.75. x 32. cylinders, 68. drivers, used 300 lbs. boiler pressure, weighed 772,000 pounds, and developed a pull of 135,375 lbs. They could negotiate curves as sharp as 20 degrees. The tender held 24,000 gallons of water and 28 tons of coal and the engine and tender weighed 1,189,500 pounds in working order. Their nickname came from the words "big boy," which had been written on the drive rods by an unknown Alco worker in the factory.

After testing and trials with the first Big Boy, the 4000s entered active service. They were used mainly during the peak fruit season from July through November, hauling heavy "red ball" produce trains over Sherman Hill. The "red balls" were high priority trains, made up entirely of Pacific Fruit Express refrigerator cars. Prior to the 4000s, it was common to see 2, 3 or even 4 engines used as helpers over the Hill. With one Big Boy on the point, Union Pacific realised significant operational and maintenance savings by reducing the number of locomotives and crews needed per train. This also allowed for time savings by not havng to stop and cut helpers in and out.

The Union Pacific ran these big locos for 21 years, mainly between Ogden, Utah and Cheyenne, Wyoming. In that time, each of the Class 1s ran up well over one million miles of service (the lowest mileage Big Boy was 4016 at 1,016,124 miles; the highest was 4006 at 1,064,625 miles). The Class 2's were only slightly behind, averaging over 800,000 miles each. The last revenue freight pulled by a Big Boy ran in July of 1959. Eight of these magnificent machines currently survive, yet sadly, none are currently in operating condition.

For more about the history of the Big Boy, visit and Age of Steam Railroad Museum.

Length (locomotive): 85' 10"
Length (tender): 47'
Wheelbase: 117' 7"
Over Couplers: 132' 10"
Top of rail to smokestack: 16' 2.5"
Driving wheel diameter: 68" (Boxpok-type)
Cylinders: 23.75" diameter with 32" stroke
Timken roller bearing axles
Articulated-type side rods
Walscherts valve gear
Max speed: 80mph
Tractive effort: 135,000 lbs. max
Max curvature: 20 deg.
Weight (locomotive): 762,000 lbs.
Weight (tender unloaded) 171,500 lbs.
Tender water capacity: 24,000 gal.
Tender fuel capacity: 28 tons
   Specs Courtesy of

Model: Jens-Christian Baerenz
Textures: Jens-Christian Baerenz, John Fowlis
Cab: Graeme "Sam Spade" Cox
Lighting: Kelley Ellison
Original Sounds and Physics: John Fowlis, Harold Clitheroe, Chuck Schneider
Packaging and Integration: Chuck Schneider

Beta Testers and Other Input
Bob Boudoin, Rob Easterday, Chris Lee, Matt Leistico, David Luke, Colin Maxted, Tom Price, John Rosh, Bob Sutherland, Steve Thompson, and many others from the MSTS community. A heartfelt "thank you" to each and every one of you.


© 2002 North American Locomotive Works. All rights reserved


All release dates subject to change without notice.

© 2002 North American Locomotive Works. All rights reserved.