The EMD SDP40F was a six-axle 3,000 hp (2.2 MW) C-C diesel–electric locomotive built by General Motors Electro-Motive Division (EMD) from 1973 to 1974. Based on Santa Fe's EMD FP45, EMD built 150 for Amtrak, the operator of most intercity passenger trains in the United States. Amtrak, a private company but funded by the United States government, had begun operation in 1971 with a fleet of aging diesel locomotives inherited from various private railroads. The SDP40F was the first diesel locomotive built new for Amtrak and for a brief time they formed the backbone of the company's long-distance fleet.

The EMD F40PH is a four-axle 3,000–3,200 hp (2.2–2.4 MW) B-B diesel-electric locomotive built by General Motors Electro-Motive Division in several variants from 1975 to 1992. Intended for use on Amtrak's short-haul passenger routes, it became the backbone of Amtrak's diesel fleet after the failure of the EMD SDP40F. The F40PH also found widespread use on commuter railroads in the United States and with VIA Rail in Canada. Additional F40PH variants were manufactured by Morrison-Knudsen and MotivePower between 1988 and 1998, mostly rebuilt from older locomotives.