EMD SD39

The EMD SD39 was introduced in August 1968 as a late addition to Electro-Motive Division's second-generation six-axle locomotive lineup, which had been substantially reorganized in 1966 around the new 645-series diesel engine. While EMD had already released the SD38, SD40, and SD45 as part of that overhaul, the SD39 filled a specific niche between the lower-powered SD38 and the more powerful SD40, offering 2,300 horsepower from a turbocharged twelve-cylinder prime mover. Production continued until May 1970, with a total of just 54 units constructed, making the SD39 one of the less common entries in the 645-era six-axle family. All units were sold to American railroads during this relatively brief production window. Because of its modest production run, the SD39 never achieved the widespread presence of contemporaries such as the SD40, which became one of the most successful diesel locomotive models in North American history. The SD39's particular power output appealed to operators who needed the tractive effort of a six-axle platform without the fuel consumption or complexity associated with higher-horsepower turbocharged sixteen- or twenty-cylinder engines. A notable variant of the type was the SDL39, ordered by the Milwaukee Road, which featured a shorter and lighter carbody design intended to reduce the weight carried per axle, making the locomotives better suited to lightly laid track. The SD39's legacy extended beyond its original owners, as a number of units were eventually resold and rebuilt for operators outside the United States, including FEPASA in South America, which acquired fourteen examples rebuilt to Dash 2 specifications and designated SD39-2s. Although EMD catalogued an official SD39-2 model following the January 1972 launch of its Dash 2 series, no new units of that designation were ever factory-built. Some existing SD39s were upgraded to Dash 2 standards and reclassified accordingly, ensuring the model's mechanical legacy persisted well beyond its original production run.

The SD39 was powered by the EMD 645E3 prime mover, a turbocharged twelve-cylinder version of the 645-series engine producing 2,300 horsepower. The locomotive rode on a C-C wheel arrangement, meaning two three-axle trucks, and employed direct-current traction motors, consistent with standard EMD practice of the period. Like all members of the 645-era six-axle family, the SD39 shared a common frame, cab structure, trucks, traction alternator, and air brake system with its siblings, the principal distinction among models being the size and configuration of the prime mover. Because the twelve-cylinder 645E3 was physically smaller than the sixteen-cylinder engines used in the SD40 or the twenty-cylinder units fitted to the SD45, the SD39's hood contained noticeably more unoccupied space, resulting in larger end platforms, or porches, and additional void space within the hood between the main generator and the electrical cabinet. This shared-component philosophy was central to EMD's strategy during this era, simplifying maintenance and parts inventory for railroads operating mixed fleets. The turbocharged twelve-cylinder configuration offered a middle ground in terms of power density, providing more output than the naturally aspirated sixteen-cylinder SD38 while placing less mechanical stress and thermal load on the engine compared to the larger turbocharged variants. The relatively small number of SD39s built suggests that most buyers found the gap between the SD38 and SD40 either unnecessary or too narrow to justify a separate locomotive type, which may explain why EMD never actively promoted the model after its initial production run concluded.