I've had to take apart many of the K-Line 21" cars, including those that Lionel issued under their own brand that were based on K-Line tooling. I agree the 6 steps John Knapp has set out above and would add a couple of points:
A. On the streamlined cars with skirting, which I believe your set has, the trucks will get hung up on the skirting when you try and pull the metal frame out unless you detach them from it. I don't believe these cars were assembled at the factory with the trucks already screwed to the frame. There is a small screw underneath the end of the truck which is closest to the center of the car. That screw secures a plate that the truck is attached to against the frame. If you remove that screw the truck should slide back; the other end of the plate is simply a tab that slides into a slot in the frame.
Below is a photo of the underside of my 21" Milwaukee Road observation car showing the screw position (green arrow under the looped ground wire):
B. Even if you've loosened everything you may have to use a shot of WD40 to get the frame to slide freely out of the channels in the aluminum body that hold it in place. I have not found that this damages any of the paint but unless you use it sparingly it is very messy to clean up. I apply it with a Q-tip at the join between the frame and the channel, which you can also see in the photo (red arrow).
The whole process is no picnic but enjoy anyway!
