Union Pacific, which usually bought its diesels by the hundreds, approached EMD’s new 3,600-hp SD45 cautiously, waiting two years after the model’s 1966 debut, then acquiring only 50. Its instincts were right: though powerful, the 20-cylinder 645E3 prime mover proved to be crankshaft-buster. Geared for 90-mph and assigned to fast freights, the units eventually were re-geared for a more sedate 65 mph and reassigned to unit coal trains. All would be off-roster within a decade. This is still in the future on October 19,1968, as the 3631 enters Denver’s 36th Street yard, just off the hill-and-dale Kansas Division with the overnight from Kansas City. The express car is a regular on this run, probably loaded with magazines and mail.
