Built in 1973/74, the SDP40F was in short a FP45 that incorporated Dash 2 technology, including the HT-C six-axle truck. On paper, the 3000-hp "F-unit" should have been a success. However, a series of passenger train derailments on curves (two degree) involving SDP40Fs cast a bad light on the six-axle EMDs, and Amtrak opted to permanently sideline the entire 150-unit fleet in 1977. The decision may have been also influenced by engine crews who had criticized the locomotive for its rough riding characteristics and severe side-to-side sway. The ride quality reportedly was far removed from that of predecessor E-units.

The SDP40F was designed during a period of U.S. Government price controls, and EMD was not inclined to introduce an entirely "new model." The SDPFs, which were essentially freight motors, became the answer. Moreover, Amtrak's longevity was being questioned by some observers; thus SDPFs could be readily sold to freight railroads in the event the inter-city passenger hauler collapsed.

Beyond questionable track conditions on some host railroads, various investigations identified three potential contributing factors for the derailments, but a definitive cause was never determined:

1. "Top heaviness" of SDP40F caused by its two steam-heat boilers, and associated internal water tank.
2. Adverse yaw of the SDP40F's hollow-bolster HTC six-axle truck.
3. Harmonics effect generated by the lead truck of a light-weight baggage car coupled directly behind a pair of SDP40Fs.

By late 1978, Amtrak SDP40Fs had been returned to EMD, scrapped or sold. Two of the returns were converted by EMD to function as component testing vehicles. Santa Fe acquired 18 of the retired SDP40Fs from Amtrak in 1984. Internally, the steam boiler and water tanks were removed, freight speed gearing installed, and hollow bolster trucks replaced. It was also necessary to improve locomotive access; new front stairway steps were added and the nose notched for increased walkway space. The modified locos were classified as a SDF40-2 and placed in service as #5250-5267. The 18 SDFs were retired by BNSF in 2002. Only one of the modified locos survived; ex-Amtrak #644 was acquired by Dynamic Rail Preservation Inc.