The EMD SDP40F was a six-axle, 3000 hp diesel passenger locomotive produced by General Motors Electro-Motive division specifically for Amtrak. It is essentially an SD40 locomotive with a cowl body and a steam generator.
Upon the formation of Amtrak in 1971, it initially inherited a number of second hand first-generation passenger locomotives, with much of it in need of replacement. Two types of locomotives were immediately identified; first, a long-ranged diesel unit equipped with large fuel tanks and water reserves to be used on long-distance journeys. The second type was a smaller, lighter unit meant to working on shorter Corridor routes. Of the two, the first was considered a priority. EMD won the contract to build 150 units, designated SDP40F.
The SDP40F was based on the proven SD40-2 locomotive. It shared the powertrain and the controls of one, but it also has a steam generator installed at the rear, for heating passenger trains. It was also built with an F45/FP45 style cowl body. Finally, EMD used the new HT-C (High-traction 3-axles truck) truck in place of the SD40's Flexicoil trucks. It should be noted that unlike later locomotive sbuilt with the HT-C truck, the trucks on the SDP40F had hollow bolsters. The first unit was delivered in 1973, with the last unit arriving in 1974.
The SDP40F was mechanically reliable, but it experienced a number of derailments, always on the trailing unit and always on shallow curves. The hollow bolsters of the truck was immediately suspected, forcing trains pulled by SDP40Fs to slow down in cuves substantially, throwing long-distance schedules out the window. Although later investigations traced the causes to the uneven weight distribution caused by the steam generators, added to a yawing effect that occured when a lightweight baggage car was coupled beind an SDP40F, the damage was done to the reputation of the SDP40F. For the second batch of F40PHs, 25 almost-new SDP40Fs were traded in and various parts reused.
These locomotives were retired from Amtrak service by 1985. Santa Fe traded a number of locomotives to Amtrak in exchange for the SDP40Fs, on a horsepower-to-horsepower basis. Before Santa Fe started using them in service, they replaced the hollow bolsters on the trucks with standard bolsters. They als removed the steam generator equipment. The SDF40-2, as it was then called, remained in service until 2001, when they were finally retired and scrapped.
One unit, Amtrak 644, is preserved in Portland, Oregon.