Hopefully your 21" aluminum cars are similar to my 15" Milwaukee Road Hiawatha aluminum cars:
1) Remove the under body detail plastic piece between the two trucks. There is a black screw at each end of this piece.
2) Again, under the body, remove the four screws, two at each end of the car. While under the car, you will see another set of four black screws. These are further toward the center of the car. Just LOOSEN them SLIGHTLY.
3) You should now be able to slowly pull the vestibules from the end of each car. Get a fingernail between the body and the vestibule and pull slowly.
4) After removing the vestibules, locate the wiring. It will be at one end only. This wiring connects the lighting in the ceiling with the wiring that goes down into the trucks and attaches to the third rail pickups. There is a plug and socket connector that needs to be unplugged before you can get to the interior. There are little "fins" on the plug and on the socket. These need to be pressed in order to release one from the other. I used two needle nose pliers and pulled gently.
5) Once you have removed the plug from the socket, you will find that K-Line hot-glued the wires to the ceiling or at least to the inside of the body (this is what I found on the Hiawatha). Use a hot glue gun to soften the glue and pull the wires out, being careful not to pull them from the ceiling fixture.
6) Now you should be able to slide the frame out of the body shell. As someone else said, be careful to keep the trucks straight so that they don't get hooked on the body. The Hiawatha had a plastic floor attached to the frame and then appropriate furniture glued to the floor. I found that there was plenty of seats to which you could glue many more "people." In fact, K-Line made only four people for this set of six cars: a man in a gray business suit, a woman with a yellow top, an African American young man in a T-shirt and a woman with a blue top. These are repeated over and over again in the various cars. I bought more K-Line people from Brasseur and painted them different colors.
(Remember to tighten those four "inner" screws on the bottom of the frame that I suggested "loosening" earlier. Those screws hold the plastic "floor" of the interior to the metal frame. By loosening them, it makes it easier to slide the frame out of the aluminum shell. When you are finished re-assembling the car, remember to tighten them again.)
Good Luck!
John