Santa Fe's SDF40-2 were originally built for Amtrak, and their original designation was SDP40F. They were first brand new locomotive units bought by the fledgling passenger service.

      Amtrak's decision to buy modern power came from the notion that the locomotives could be converted into freight engines should Amtrak fail. Amtrak gave the nod to EMD, although GE had produced the U30CG. The specifications given to EMD were to produce a locomotive similar in outward appearance to the FP45, however would have the interior components of the SD40-2. Amtrak also opted for the more economical 16-cylinder engine instead of the 20-cylinder engine, thus adding better fuel economy and reliability to the new class of locomotives.

      The new SDP40F locomotives were delivered in 1973 and 1974 to Amtrak. The company pressed them into cross-country service almost immediately after delivery. Amtrak looked favorably on the new locomotives, and was planning on retiring its hand-me-down and older engines now that new power was finally at hand.

      Things continued to look upward for Amtrak and the new SDP40Fs until 1976. Almost from the start, train crews commented to management about the "yawing" of the trucks which were causing rough riding and minor derailments. Later on in that year, both the Burlington Northern and the Louisville and Nashville suffered serious derailments on 2-degree curves involving the SDP40F. The Burlington Northern decreed that no SDP40F would run on its rails after the incident. The company relented only after a Federal Rail Administration ruling subjecting the SDP40F to speeds no greater than 40 MPH on 2-degree or greater curves.

      Most of the blame for the series of accidents was pointed to the SDP40F's unique hollow-bolster HTC truck. Many tests were conducted to try to prove if the truck was indeed at fault, however no test was ever conclusive.

      Amtrak's image was beginning to suffer immensely because of the major problems with the SDP40F, however Amtrak was at the same time beginning to receive a four-axle F40PH locomotive, and the decision was made to replace the SDP40F with the F40PH.