When this era TMCC diesel was manufactured, Lionel designed what I refer to as the "PITA" truck assembly.
The motor actually screws into a mounting plate from the bottom; unfortunately, you have to remove that mount plate from above. I haven't done mine recently, but the screws that hold both pickup rollers will need to come out along with any other forgotten screws I'm over-looking from memory....
You'll see the mounting plate in question once you remove the shell and get some visual access.
