EMD's "50 Series" diesel locomotives of the mid-1980s were the product of the most extensive testing in the company's 50-year history. Both the four-axle, four-motor GP50 and the six-axle, six-motor SD50 utilized a new control system that substantially increased hauling capacity and reduced engine noise levels.

The 3500 hp SD50 measured just 30" longer than the SD40-2 it replaced but differed substantially in appearance. Gone were the front and rear "porches" of the SD40-2 and the dynamic brake blisters on the sides of its hood. Under the SD50's longer hood, the dynamic brakes, which generate a lot of heat, were relocated from a position over the hot engine to a cleaner, cooler area just behind the cab.

The SD50 also introduced EMD's new "Super Series" control system, which included a radar unit for measuring true ground speed and electronic monitoring of the electrical equipment. A primary benefit of the new control system was its ability to maximize rail adhesion by automatically applying short bursts of sand as needed to maintain traction. The system allowed three SD50s to do the work of four older locomotives.