EMD DD35A

The EMD DD35A was a cab-equipped diesel-electric locomotive built by General Motors Electro-Motive Division exclusively for the Union Pacific Railroad in 1965. It represented a direct evolution of the earlier cabless DD35 booster unit, which Union Pacific had acquired to satisfy its appetite for concentrated horsepower on its demanding main lines across the American West. Fifteen DD35A units were constructed between May and July of 1965, receiving Union Pacific road numbers 70 through 84. The type entered service alongside other experimental high-horsepower locomotives of the era, including the ALCO Century 855 and the GE U50, all of which emerged in response to Union Pacific's aggressive pursuit of greater tractive effort from fewer locomotive units. Early service life was troubled by reliability problems that plagued the DD35 family generally. Among the issues identified was contamination of electrical equipment by sand from internal sandboxes, a problem serious enough that new external sandboxes were mounted on the walkways during a 1969 modification program. The DD35A units were also among the last EMD road locomotives built with DC generators and conventional switchgear, equipment that demanded more maintenance attention than the alternator-based systems EMD introduced on subsequent product lines. Once these early difficulties were addressed, the units performed adequately in heavy road service, though their considerable length and limited versatility made them less adaptable than smaller, more numerous consist combinations. The economic pressures of the early 1980s accelerated the retirement of large, specialized units like the DD35A. By that point their operational inflexibility outweighed the horsepower advantage they offered, and Union Pacific withdrew them from service, reportedly concentrating their final assignments around Salt Lake City, Utah before retiring the class entirely by approximately 1981. No examples survived into preservation. Despite their brief service lives, the DD35 family served as an important intermediate step toward the DDA40X, the massive twin-engined locomotive that Union Pacific introduced in 1969 and which became the most celebrated product of this lineage.

The DD35A was in essence two EMD GP35 power assemblies united on a single extended frame, with each unit contributing one 567D3A prime mover for a combined output of 5,000 horsepower. The locomotive rode on two four-axle Flexicoil trucks, giving it a D-D wheel arrangement that spread its considerable weight across eight powered axles. Unlike the cabless DD35, the DD35A required a longer frame to accommodate a full GP35-style control cab at the front end, and the fuel tank beneath the carbody was enlarged accordingly. The central walkway running the length of the locomotive was also offset slightly rearward compared to the booster version to account for the space occupied by the cab structure. One notable design feature carried over to the DD35A from EMD's concurrent experimental work was the adoption of a flared radiator section at the rear of the carbody, a configuration EMD was simultaneously evaluating on its early 645-engined demonstrator locomotives, including prototype SD40 units. This detail linked the DD35A to the transitional period in EMD's engineering history between the 567 and 645 engine families. The drivetrain relied on DC traction motors and conventional generator equipment throughout, reflecting its mid-1960s design origins, and this older electrical architecture contributed to the maintenance demands that characterized the type during its operational career.

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