The Metra F40Cs combined the frame, HT-C trucks, and machinery of the SD40-2 with a shortened SDP40F cowl and a HEP alternator; the SDP40Fs had a longer frame to accomodate a pair of skid-mounted steam generators for train heating (which could be - and later were - replaced with skid-mounted HEP generator sets) and special hollow-bolster HT-C trucks.

There was some speculation that the hollow-bolster trucks, combined with the SDP40F's truck-center spacing and weight distribution, led to yawing problems that may have caused the many derailments these units were involved with. Unfortunately, commuter train operators apparently felt the F40C had the same potential for trouble even though the F40C was essentially a cowl and HEP-equipped SD40-2, and was thus unlikely to suffer the same problems (which has long since proven to be the case); the F40PH became the preferred choice for commuter service and Metra's F40Cs were the only ones ever built.