The SD45 is a six-axle diesel-electric locomotive class built by General Motors Electro-Motive Division between 1965 and 1971.

Reliability was not as high as anticipated; the twenty-cylinder prime mover was prone to crankshaft failure from engine block flex.

The SD45X trialed several new features, including a more powerful EMD 645E3 engine producing up to 4,200 horsepower (3,130 kW) and a newly designed truck intended to have higher adhesion.

Changes to the body included the use of different radiator fans and a flat rather than beveled end to the long hood.

The Southern Pacific Transportation Company rebuilt a total of 167 EMD SD45 units into EMD SD45R diesel locomotives at their own Sacramento Shops under the Southern Pacific's M-99 rebuild program and renumbered their units as 7400 through 7566.