8 Surviving 4-8-8-4 Union Pacific Big Boy Locomotives Click on thumbnail for full-size view
Big Boy #4004 Holliday Park Cheyenne, WY	Big Boy #4005 Forney Transportation Museum Denver, CO	Big Boy #4006 Museum Of Transportation St. Louis, MO	Big Boy #4012 Steamtown National Historic Site Scranton, PA
Big Boy #4014 Los Angeles County Fairplex Pomona, CA	Big Boy #4017 National Railroad Museum Green Bay, WI	Big Boy #4018 Age Of Steam Museum Dallas, TX	Big Boy #4023 Kenefick Park Omaha, NE

During the late 1930s, the Union Pacific often used helpers to move trains from Ogden to Wasatch.

The UP wanted to simplify this move so they asked their "Department of Research and Mechanical Standards" to design a locomotive that could pull a 3600 ton train unassisted over the 1.14% grade of the Wasatch. 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.