The Union Pacific (UP) Railroad’s eastbound main from Ogden, Utah to the peaks of the Wahsatch Mountains near the Wyoming border rise 2,500’ over a distance of 65 miles creating a grade of 1.14%. During the late 1930s, the UP often used helpers to move trains from Ogden to Wahsatch. The UP wanted to simplify this move so they asked their "Department of Research and Mechanical Standards" (DoRMS) to design a locomotive that could pull a 3,600 ton train unassisted over the 1.14% grade of the Wahsatch.
The designers determined that to pull a 3,600 ton train, a tractive effort of 135,000 lbs would be needed. Assuming a factor of adhesion of 4.0, the weight on drivers would have to be 4.0 X 135,000 = 540,000 lbs. Given an axle loading of 67,500 lbs each, this would require 8 drivers or an x-8-8-x wheel arrangement. The designers agreed upon the 4-8-8-4 design. Next, the horsepower and cylinder sizes were computed based on 300 psi boiler pressure. Although they weren't planning to pull these freight trains at 80 MPH, the DoRMS designed them for 80 MPH in order to have a sufficient factor of safety built into the design. What resulted is considered by many to be the most successful articulated steam locomotive ever built. The first Big Boy Locomotive (#4000) was delivered to Omaha, Nebraska on September 5, 1941.
The 25 Big Boys were built in two groups. The first group, called "class 1", was built starting in 1941. They were numbered 4000-4019. The second group, "class 2", was built in 1944. They were numbered 4020-4024. The last revenue freight pulled by a Big Boy was in July of 1959. Most were retired in 1961. The last one was retired in July of 1962. As late as September, 1962, there were still four operational Big Boys at Green River, Wyoming.
Books and magazines will often state that the Big Boy was the largest steam locomotive ever built. When steam locomotives are compared, many different quantities can be considered. For example, weight, length, horsepower, and tractive effort are all characteristics of how "large" a steam locomotive is. In each of the above categories, a locomotive "larger" than a Big Boy can be found.
If you were to ignore all of the unsuccessful and experimental locomotives, then look at the overall ratings of the remaining steam locomotives in the above categories, it would become clear that the Big Boys (along with the N&W Y6 and A, DM&IR M, and perhaps the C&O H-8 locomotives) were the "largest" among all successful steam locomotives.
Unlike many famous locomotives that were built in great quantities but left no survivors, eight Big Boys have been preserved from the scrapper’s torch. Today, seven are on display in cities throughout the U.S. In 2013 Union Pacific brought No. 4014 out of retirement from the Rail-Giants Museum in Pomona, California and restored it before it returned to service in 2019.
Facts and General Characteristics of the Big Boy Locomotive:
Builder: American Locomotive Company (Alco)
Cost: $265,174 ea.
Number built: 25
Length (engine and tender): 133'
Weight: 1,208,000 lbs.
Wheel Configuration: 4-8-8-4 articulated
Gauge: 4' 8 ½"
Fuel: 28 tons coal
Water: 25,000 Gallons
Cylinders: (4) 23.75" x 32"
Diameter of Drivers: 68"
Steam Pressure: 300 PSI
Power: 6,000 horsepower
Tractive Effort: 135,375 lbs.
Speed: 80 mph max, 35-40 mph for freight-hauling
Facts and General Characteristics of the Model:
Manufacturer: Monogram
