This was accomplished with a lengthened SD40 frame of 65-feet, 8-inches compared with a frame of 59-feet, 2-inches for a standard GP40. Prior to their sale to Amtrak, their air system was modified ...

The first new diesel, the six-axle SDP40F (a variation based on the venerable SD40-2) wasn't a success. Fortunately Amtrak was thinking ahead and had a design for a shorter-range four-axle diesel ...