For much of the twentieth century, the Union Pacific had a love affair with massive, larger-than-life locomotives, custom-built for the UP and rostered nowhere else. The family line that began with the Union Pacific-type 4-12-2 steamer and continued with the Challenger, Big Boy, and gas turbines ended with the DDA40X Centennials. Built by the Electro-Motive Division of General Motors in 1969-71, the Centennials are still the longest (98') and most powerful (6600hp) diesels ever built in the western hemisphere, and the last unique UP motive power. For the decade of the 1970s, they ruled the road from the Midwest to the west coast, hustling UP fast freights at up to 90mph.
In the early 1960s, the UP had asked the major diesel builders to come up with a high-powered, fast freight engine, and EMD, GE, and ALCo had each built samples following the same concept: two diesel motors on one long frame with a total of about 5000 horsepower. While ALCo's smoky, somewhat-unreliable Century 855 never made it past the prototype stage, EMD's DD35 and GE's U50 generated modest orders from the railroad. When the UP decided to revisit the concept a few years later, EMD was ready with a better idea: the DDA40X ("X" for experimental) essentially packed two GP40s under one hood, incorporated the then-new wide-nosed cab for increased crew comfort and safety, and served as a test bed for the electronics that EMD would later incorporate into its next generation of diesels, the Dash 2 series. The first engine, no. 6900, was rushed to completion in time for the May 10, 1969 centennial of the golden spike that completed the first transcontinental railroad, giving the new class of diesels its name. The 47 Centennials became the UP's premier power on its western main lines and kept that crown for a decade, with many units running up more than 2 million miles of service.
By 1980, however, the units were heavily worn, and a traffic downturn that year led the UP to mothball the Centennial fleet. Although many returned to service briefly in 1984-85, the now-failure-prone engines were soon retired in favor of younger, more modern power. But the giant engines had acquired quite a following among railfans, one of whom spray painted "Save the Whales, Save the 6900s" on several highway viaducts near UP's Salt Lake City yard. In response, the railroad donated or preserved a dozen Centennials and kept no. 6936 on its active roster, where it still serves today on special business trains, promoting the railroad to current and prospective shippers.
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