"During the late 1930s, the Union Pacific 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 3600 ton train unassisted over the 1.14% grade of the Wahsatch.
The designers determined that to pull a 3600 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 * 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. 4000 was delivered to Omaha at 6PM, September 5, 1941.
The 25 Big Boys were built in two groups. The first group, called "class 1", were built starting in 1941. They were numbered 4000-4019. The second group, "class 2", were 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, WY.
The total mileage of each of the Big Boys from class 1 were roughly the same -- 1,000,000 miles. 4016 had the lowest mileage -- 1,016,124. 4006 had the highest mileage -- 1,064,625. Of the second group, 4024 had the highest mileage -- 811,956.
Are Big Boys Really the Largest Steam Locomotive?
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." (excerpted from (
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"Even in retirement, Old Number 4004 remains an imposing sight. The world's largest steam locomotive, this powerful coal-fired engine was designed to pull a 3600-ton train over steep grades between Cheyenne, WY and Ogden, Utah. The 4004 is one of the eight remaining Big Boys on display throughout the country.
On display year-round in Holliday Park. The engine is in the southeast corner of the park. You can park your car in the lot accessible from East 17th Street and Morrie Avenue." (from (
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