The EMD MP15AC is a 1,500 hp (1,120 kW) diesel

Diesel locomotive

A diesel locomotive is a type of railroad locomotive in which the prime mover is a diesel engine, a reciprocating engine operating on the Diesel cycle as invented by Dr. Rudolf Diesel...

 switcher

Switcher

A switcher or shunter is a small railroad locomotive intended not for moving trains over long distances but rather for assembling trains ready for a road locomotive to take over, disassembling a train that has been...

/road-switcher locomotive

Locomotive

A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

 built by General Motors' Electro-Motive Division between August 1975 and August 1984. 246 examples were built, including 25 for export to Mexico

Mexico

The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...

, and four built in Canada

Canada

Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...

. The MP15AC was a development of the earlier MP15DC

EMD MP15DC

The EMD MP15DC was a switcher-type diesel locomotive model produced by General Motors' Electro-Motive Division between 1974 and 1980. 351 examples were built...

 model, but replaced the earlier model's DC

Direct current

Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...

 generator

Electrical generator

In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...

 with an alternator

Alternator

An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.Most alternators use a rotating magnetic field but linear alternators are occasionally used...

 producing AC power

Alternating current

In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....

 which is converted to DC for the traction motors with a silicon rectifier

Rectifier

A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...

. 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 its predecessor. 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 its earlier DC sisters.

Railroads that originally bought MP15ACs were the Southern Pacific

Southern Pacific Railroad

The Southern Pacific Transportation Company , earlier Southern Pacific Railroad and Southern Pacific Company, and usually simply called the Southern Pacific or Espee, was an American railroad....

, Missouri-Kansas-Texas

Missouri-Kansas-Texas Railroad

The Missouri–Kansas–Texas Railroad was incorporated May 23, 1870. In its earliest days the MKT was commonly referred to as "the K-T", which was its stock exchange symbol; this common designation soon evolved into "the Katy"....

 (MKT), TFM

Grupo Transportación Ferroviaria Mexicana

Kansas City Southern de México , formerly Transportación Ferroviaria Mexicana , is the name of a company dedicated to freight transportation using rail in the North Eastern part of Mexico...

, Conrail, Long Island Rail Road

Long Island Rail Road

The Long Island Rail Road or LIRR is a commuter rail system serving the length of Long Island, New York. It is the busiest commuter railroad in North America, serving about 81.5 million passengers each year. Established in 1834 and having operated continuously since then, it is the oldest US...

 (LIRR), and the Milwaukee Road. Former Milwaukee Road units are now owned by the Soo Line Railroad

Soo Line Railroad

The Soo Line Railroad is the primary United States railroad subsidiary of the Canadian Pacific Railway , controlled through the Soo Line Corporation, and one of seven U.S. Class I railroads. Although it is named for the Minneapolis, St. Paul and Sault Ste...

 (an American operating subsidiary of the Canadian Pacific Railway

Canadian Pacific Railway

The Canadian Pacific Railway , formerly also known as CP Rail between 1968 and 1996, is a historic Canadian Class I railway founded in 1881 and now operated by Canadian Pacific Railway Limited, which began operations as legal owner in a corporate restructuring in 2001...

); 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

Wisconsin and Southern Railroad

The Wisconsin and Southern Railroad is a Class II regional railroad in the southern portion of Wisconsin and the northeast corner of Illinois. It operates former Chicago, Milwaukee, St...

 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 Railroad MP15ACs to haul freight along with other ex LIRR locomotives.