Starting in May 1969, Union Pacific operated Centennial diesel-electric locomotives, famous as the largest diesel-electric locomotives ever built.
Built by what was then General Motors Electro-Motive Division (EMD), the units — officially the EMD DDA40X — were nicknamed in honor of the centennial anniversary celebration of the completion of the continental railroad.
All 47 of the Centennial locomotives were delivered to UP by 1971. The locomotives were fully retired by 1984.
The locomotives were numbered from 6900 to 6946, in honor of the 1869 Golden Spike ceremony in Utah; the first of the locomotives attended the Golden Spike Centennial celebration.
The units were two engines built on a single frame. The locomotive weighed a stunning 270 tons, measured 98 feet long and delivered 6,600 horsepower.
No. 6936 was assigned to special service in 1985 and remains in use by the railroad. Today, the locomotive is part of the UP Heritage Fleet.