Union Pacific, one half of the first transcontinental railroad, became famous for large, powerful locomotives, a tradition that continued into the diesel era with the DDA40Xs, custom-built for the railroad and introduced in 1969 as the largest and highest-horsepower diesels ever built. These 270-ton, 98-foot-5-inch locomotives produced 6600 horsepower from two 16-cylinder diesel engines, could reach 90 miles per hour, and before 1980 averaged more than 200,000 miles per year. Built by the Electro-Motive Division of General Motors, they also served as prototypes for redesigned electrical systems and other advances that appeared in new locomotives in 1972. Called “Centennials” in honor of the 100th anniversary of the Golden Spike Ceremony of May 10, 1869, which completed the transcontinental railroad, 47 units numbered 6900 through 6946 were built from May 1969 to September 1971. They operated on Union Pacific main lines across the American Midwest to Portland, Oregon, and Los Angeles, California, and for a short time after the Western Pacific merger a few ran on the former Western Pacific main line from Salt Lake City to Oakland; crews also called them “Big Jacks.” By 1986, newer diesel models and an economic downturn brought the Centennials’ retirement. Union Pacific kept unit 6936 for its historic fleet and donated 12 others for preservation. Unit 6946, built in September 1971, was the last Centennial built and the first donated. With 6600 horsepower, a top speed of 90 miles per hour, an operating weight of 545,432 pounds, and an original cost of $551,168.00, it came to hold iconic status in the railroad world.