This is the Lionel TMCC Dash 9 #6-18289 that came with the 2002-2003 CN Tank Train set.
The motor mount screws had loosened off and dropped out of the motor, allowing the motor to simply freewheel instead of driving the truck. This first pic shows the swivel D plate where the motor mounts. This plate retains the truck in the frame and is generally fastened to the truck by a single machine screw which comes up from the bottom of the truck. You can see the two motor mount screw holes on either side of the larger center hole, which is where the motor worm gear inserts. The motor simply came off in my hand when I pulled up on it. Yikes!
This next pic shows the truck underside and the head of the screw that fastens the truck to the upper D plate. The assembly of the power truck and motor is different than anything I have previously seen, and defies all logic. Note that the screw head is barely accessible using a TINY thin shafted Philips screwdriver. Ordinarily you would unfasten the power pickup that is in the way by undoing a small machine screw, thus leaving easy access to the truck screw. Guess what? The screw fastening this pickup is installed from the TOP side of the truck, and its head is totally inaccessible until you get the truck removed from the motor. I wonder what genius thought this arrangement up?
I was able to get the truck mount screw loose after I found a small enough Philips, but even with it loose, the upper swivel D plate does not seem to want to come off the truck at all. I have backed the screw off to the point where it's head is contacting the power pick up pin that is right above it. I have tried judiciously prying the truck and the D plate apart from the top side, with no luck at all. Has anyone seen this style truck and figured out how you get it apart? Thanks for any help!