EMD MP15AC

The EMD MP15AC was a diesel road switcher locomotive produced by General Motors' Electro-Motive Division between August 1975 and August 1984. It emerged from an evolution of the earlier MP15DC, which had demonstrated that a switcher-sized locomotive riding on standard Blomberg B trucks could operate at road freight speeds of up to 60 miles per hour. Railroads that had already invested in EMD's alternator-equipped road locomotives through the 1970s found the MP15AC a natural fit for their maintenance programs, as its electrical systems were similar to those already in service across their fleets. Over the course of its production run, 351 examples were constructed, with customers including both domestic American railroads and buyers in Mexico and Canada. The six largest purchasers of the MP15AC were railroads already familiar with EMD's AR10 alternator technology through their road locomotive acquisitions, making parts commonality and crew familiarity significant practical advantages. An additional 36 units found their way to eight smaller customers. The model's production span of nearly a decade reflected sustained demand for a capable, versatile locomotive that could bridge switching and light road freight duties, a role that purely dedicated switchers of earlier generations could not as easily fulfill. The MP15AC represented a meaningful step in the broader standardization of diesel-electric locomotive technology, as the alternator-rectifier arrangement it employed gradually displaced the older DC generator as the accepted approach for new designs throughout the industry. Its service life on many railroads extended well beyond its production years, and examples could be found active into the twenty-first century on short lines and regional carriers that valued its combination of moderate power and mechanical simplicity.

The MP15AC was powered by a roots-blown, 12-cylinder version of EMD's 645E prime mover, rated at 1,500 horsepower. The 645 series engine had been introduced in 1966 and served as EMD's standard power plant through the 1980s, lending the MP15AC a well-understood and proven mechanical foundation. The locomotive rode on a B-B wheel arrangement, meaning two two-axle powered trucks, with all four axles driven by DC traction motors fed through a silicon rectifier that converted the output of the main alternator from alternating to direct current. This alternator-rectifier system distinguished the AC variant from the DC-generator-equipped MP15DC and proved more reliable in service. Physically, the MP15AC measured approximately 1.5 feet longer than its MP15DC counterpart, the additional length being required to house the rectifier equipment. Its cooling system also differed visibly from earlier EMD switcher designs. Rather than the front-mounted radiator intake and belt-driven cooling fan that had characterized previous models, the MP15AC used intakes positioned on the lower sides of the forward nose section combined with electrically driven fans. This arrangement allowed the locomotive to draw in cooler ambient air from a lower position and eliminated the reliability problems associated with the belt-driven fan systems found on earlier switchers. These engineering refinements made the MP15AC both more dependable in day-to-day operation and more representative of the direction that diesel-electric locomotive design would take in subsequent decades.

Models by: Athearn · Walthers