After testing EMD’s GP35-DD35-DD35-GP35 demonstrator set, Union Pacific was so pleased with the design that it almost immediately ordered an additional 25 DD35s and 22 matching GP35s, which were delivered between May and August 1964. These joined the four-unit demon¬strator set previously acquired by the carrier.
As with UP’S earlier experiences with cabless booster units, the lack of flexibility of these massive, mainline-only units soon be¬came apparent, and a planned follow-on order for 15 units was changed from booster units to 15 units with fully equipped cabs – the DDA35 was born.
The Centennials were the final example of Union Pacific’s search for the ideal loco¬motive. While the road pursued a study of its motive power needs in late 1967 and early 1968, UP in March 1968 tried, as a stopgap measure, using EMD’s SD45 model as a high-horsepower, high-speed locomotive, but with limited success. UP returned to the concept of double-engine locomotives and in collaboration between the railroad and EMD, the DDA40X came to life. Within 13 months of an initial March 1968 communication, the railroad and the builder had worked out a definitive design, and the first unit was delivered. The timing of the first unit coincided with the 100th anni¬versary of the May 10, 1869, ceremony marking the driving of the Golden Spike at Promontory Summit, Utah – the DDA40X quickly became known as the Centennial. It was numbered 6900, and 46 more examples arrived over the next 28 months.

128 pages, 8-1/2x11-inch vertical-format perfect-bound (softcover) book with more than 130 black-and-white and 170 color photos, as well as diagrams.