I have found MTH cars the most difficult to Kadee.
I have tried complete disassembly. As Hot Water mentioned, this leads to the unhappy "springs flying all over the room" syndrome. (Wear eye protection! )
I came to prefer using a Dremel to cut away the claw over the "Great Spring Hunt."
Now, I just prefer to buy Lionel's new cars with the trucks that are so hated by the non-Kadee crowd.