The EMD MP15AC is a 1,120 kW (1,502 hp) diesel switcher/road-switcher locomotive built by General Motors' Electro-Motive Division between August 1975 and August 1984. A variant of the MP15DC with an AC transmission, 246 examples were built, including 25 for export to Mexico, and four built in Canada.
The MP15DC’s standard Blomberg B trucks were capable of transition and road speeds up to 60 mph (97 km/h), allowing use on road freights. Soon there was a demand for a model with an advanced AC drive system. The MP15AC replaced the MP15DC’s DC generator with an alternator producing AC power which is converted to DC for the traction motors with a silicon rectifier. The MP15AC is 1.5 ft (457 mm) longer than an MP15DC, the extra space being needed for the rectifier equipment. The alternator-rectifier combination is more reliable than a generator, and this equipment became the standard for new diesel-electric locomotive designs.
The MP15AC is easily distinguished from the DC models. Instead of the front-mounted radiator intake and belt-driven fan used on all previous EMD switchers, these have intakes on the lower forward nose sides and electric fans. Side intakes allowed the unit to take in cooler air, and the electric fans improved a serious reliability issue found in its earlier DC sisters.
The MP15 used a 12-cylinder version of the 645E series engine developing 1,500 hp (1,120 kW) at 900 rpm Introduced in the SW1500, this was a supercharged 2 stroke 45 degree V type, with a 9 in × 10 in (229 mm × 254 mm) bore x stroke, giving 645 cubic inches (10.57 L) displacement per cylinder. The 645 series, introduced in 1966, was EMD’s standard engine through the 1980s. 
In the early 1970s railroads were starting to convert to AC power, the six largest buyers, Milwaukee (64), Southern Pacific (58), Seaboard Coast Line Railroad (45), Nacionales de México (25), Long Island (23), and Louisville & Nashville (10), were all buying AC road locomotives. 36 more units were sold to 8 other customers.
Former Milwaukee Road units are now owned by the Soo Line Railroad (an American operating subsidiary of the Canadian Pacific Railway); those not painted in the Canadian "Golden Beaver" scheme have worn a Soo Line patch job; those wearing it are often called "Bandits". Six former Milwaukee units returned to "home rails" in 2008, serving the growing regional Wisconsin & Southern Railroad WSOR in Milwaukee, Madison, and Horicon. In addition, Union Pacific has bought many examples on the used locomotive market. The New York & Atlantic Railway, which carries freight on Long Island, uses 4 former Long Island Rail Road MP15ACs to haul freight along with other ex-LIRR locomotives. Two units sold new to the DOE at Hanford, Washington are now in operation as Tri City Railroad #16 and #15. The Knoxville and Holston River Railroad also owns a unit.
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- Pinkpank, Jerry A (1973). The Second Diesel Spotter’s Guide. Kalmbach Books. p. 26. LCCN 66-22894.
- Sarberenyi, Rob (2013). "UtahRails.net EMD MP15DC, MP15AC, and MP15T Original Owners =". Don Strack. Retrieved 5 August 2013.
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- Marre, Louis A. & Pinkepank, Jerry A. (1989). The Contemporary Diesel Spotter's Guide. Waukesha, WI: Kalmbach Books. ISBN 0-89024-088-4. LCCN 88083625. OCLC 19959644.
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- Sarberenyi, Robert. EMD MP15DC, MP15AC, and MP15T Original Owners