and powerful locomotives such as the Big Boy, Challenger, and Turbines; yet the release of the leviathan DDA40X would draw that tradition to a close. Union Pacific was no stranger to twin- engined diesels, having employed a fleet of streamlined EMD E-units to haul its famous passenger trains, and later encouraging the development of the DD35 freight diesel in 1963. In all, 47 Centennials were built and put into service between June 1969 and Sep- tember 1971, carrying road numbers 6900 through 6946.


LEFT: Four equals five in this consist —five SD40-2s, that is. Reactivated for service after three and a half years in storage, twin-engine DD40AX Centennial 6934 roars eastbound over the summit of Cajon Pass on March 28, 1984. ABOVE: Three will get you five in this “Centennial Sandwich” with UP 6928 leading westbound at Frost on the east side of Cajon Pass on December 5, 1976. BELOW: After attempting some engine performance adjustments the brakeman is carefully making his way back to the warmth and safety of the lead SD40-2 at Martinez Spur on January 8, 1977. This Centennial Sandwich alternates layers of SD40-2s with DDA40X’s for extra flavor.


All of the Centennial units were equipped to operate at 90 miles per hour, and were assigned to UP’s high- speed priority freights. About halfway through the ten-plus years the Centen- nial fleet saw continuous service, traf- fic levels were growing but operational costs were being fine tuned to optimize the fewest number of crews required to operate a number of priority trains. Tonnage per train continued to in- crease to a point where operating schedules were beginning to suffer from a lack of sufficient power. Despite


Sandwich 45


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