The SDP45 is a six-axle, C-C, 3,600-horsepower (2,680 kW) diesel-electric locomotive built by General Motors' Electro-Motive Division of La Grange, Illinois.

It was a passenger-hauling version of the SD45 on a stretched locomotive frame with an extended, squared-off long hood at the rear, aft of the radiators, giving space for a steam generator for passenger train heating.

The Southern Pacific Railroad ordered their ten on May 9, 1966, with the units being placed in service between May 24 and July 26, 1967, initially on the City of San Francisco between Oakland and Ogden, and eventually used system-wide.

Intended only for freight service, these units had standard (angled) long hood ends, and the extra space aft of the radiators had concrete ballast where a passenger unit would have a steam generator and venting.

[1] One Burlington Northern Railroad SDP45 6599 was retrofitted with an articulated four-axle truck by EMD in 1983–84, converting it to an A1A-B+B wheel arrangement.

This testing was not related to the development of the HTCR three-axle radial truck first seen under EMD SD60s and SD60MACs and made standard on the early SD70 series.

In August 2018, Youngstown Steel Heritage announced their intention to purchase 3639 and move it to their museum, with the goal of restoring it to operating condition, and eventually back to its original number and paint scheme.