EMD DD35

The EMD DD35 emerged from a dialogue between Union Pacific Railroad and General Motors' Electro-Motive Division in the early 1960s, when Union Pacific expressed interest in extremely high-horsepower locomotive sets capable of replacing its gas turbine fleet. Union Pacific envisioned a three-unit combination producing 15,000 horsepower, but EMD's solution took a different form. The division constructed a demonstrator set in September 1963, finished in a red and white paint scheme, consisting of a pair of DD35 cabless boosters sandwiched between two GP35 cab units. This arrangement effectively combined the output of six GP35 prime movers across four physical units. The demonstrator was tested on several railroads before Union Pacific acquired the entire set and subsequently ordered 25 additional DD35 units, delivered between May and September 1964 and numbered 74B through 98B, with the two demonstrator locomotives carrying the numbers 72B and 73B. Southern Pacific also purchased the DD35, though its commitment was far more cautious. The railroad acquired only three units, numbered 8400 through 8402, which were equipped with plain bearings salvaged from F3B trade-in locomotives rather than roller bearings. Southern Pacific put the trio to work on its Sunset Route in the early years of their service lives, but the units proved to be among the first locomotives set aside during traffic downturns. They were eventually reassigned to transfer duties between West Colton and yards in the Los Angeles basin, particularly Taylor Yard. Southern Pacific scrapped all three in 1978 when their leases expired, having never ordered additional examples. The DD35 fleet experienced considerable reliability problems in its early years, with sand from internal sandbox installations contaminating electrical equipment being identified as a significant contributing factor. Revised sandboxes mounted on the exterior walkways were retrofitted in 1969 to address the issue. Once these problems were resolved, the locomotives performed adequately, though their sheer size and inflexibility made them less adaptable than smaller units during periods of reduced traffic. Union Pacific retired its original DD35 B units by 1977, and all examples were ultimately scrapped. The type nonetheless represented an important step in the development of very high horsepower diesel motive power in North America.

The DD35 was a cabless booster unit rated at 5,000 horsepower, achieved by mounting two EMD 567D3A prime movers on a single elongated frame in a back-to-back configuration. Each engine drove its own set of electrical equipment and powered one of the locomotive's two four-axle Flexicoil trucks, giving the unit a D-D wheel arrangement. This differed fundamentally from the approach used in conventional road units, where a single prime mover powered all driven axles through a common generator. The use of eight powered axles in a single unit concentrated a considerable amount of weight and tractive effort, but the rigidity of the four-axle trucks was considered potentially damaging to track, which is one reason the locomotive was intended to operate as a mid-train unit rather than at the head end. The DD35 was built with direct current generators and conventional switchgear rather than the alternator-based electrical systems that EMD was beginning to introduce on other contemporaneous designs. This older electrical architecture required more intensive maintenance compared to the AC/DC systems that would become standard on later EMD products. The locomotives used DC traction motors throughout, consistent with standard road unit practice of the era. Southern Pacific's three units were further distinguished by their use of plain journal bearings sourced from traded-in F3B units, rather than the roller bearings that would have been specified on a new order, which likely contributed to their comparatively troubled service record on that railroad.

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