EMD SD60MAC

The EMD SD60MAC emerged in 1991 as a small but historically significant variant of the SD60 family, produced in a single batch of just twenty units. These locomotives were built as demonstrators intended to validate the commercial viability of alternating current traction technology on a platform that operators already knew and trusted. The Burlington Northern was the railroad that hosted testing of these four demonstrator units, providing the operational environment necessary to prove that AC traction motors could perform reliably in heavy North American freight service. Despite the success of the testing program on Burlington Northern's demanding routes, the SD60MAC never attracted follow-on orders in any substantial quantity. The demonstration essentially served as a bridge between the older direct current traction technology that had dominated American diesel locomotive design for decades and the new generation of AC-equipped power. Operators who were persuaded by the performance results of the SD60MAC trials directed their subsequent purchasing decisions toward the EMD SD70MAC, which offered AC traction motors combined with the higher output and other improvements of the newer SD70 platform. The legacy of the SD60MAC is therefore less about the type itself than about what it proved possible. The twenty units built represent one of the smallest production runs in the SD60 family, yet their influence on the direction of American locomotive procurement in the 1990s was considerable. The SD70MAC went on to become one of the defining locomotives of that decade, particularly on heavy coal and grain hauls, and the groundwork laid by the SD60MAC trials was a meaningful part of that story.

The SD60MAC shares its fundamental mechanical architecture with the SD60M, including the full-width short hood and North American safety cab configuration. It rides on a C-C wheel arrangement, meaning two three-axle powered trucks, giving it six driven axles in total. The locomotive is powered by the EMD 710G3B prime mover, a sixteen-cylinder two-stroke diesel engine producing 3,800 horsepower. The defining characteristic that sets the SD60MAC apart from its sibling variants is the use of AC traction motors rather than the conventional DC motors found throughout most of the SD60 series. AC traction motors offer advantages in adhesion and tractive effort at low speeds, making them particularly well suited to heavy drag freight operations on grades. The electrical system required to support AC traction is more complex than a conventional DC arrangement, as it requires an inverter to convert the alternating current generated by the main alternator into the variable-frequency AC power that the traction motors need at different speeds. This added sophistication was part of what the Burlington Northern trials were designed to evaluate under real working conditions. The 710G3B engine used in the SD60MAC is the same basic powerplant later employed in the SD60E rebuilds carried out by Norfolk Southern, though in that application it was tuned to produce 4,000 horsepower.

—