The kadee ready lionel cars are the easiest. Just unscrew the lobster claw from the truck, put the kadees on and go. They come with a pad, but sometimes the pad makes the coupler sit too low. For Lionel cars without pre-drilled holes, there are a lot of available 3D printed pads available for various cars that will properly locate the coupler and aid in drilling the holes.
Recent MTH cars are pre-drilled, but you usually have to completely take the car apart to remove the truck to get the old coupler off. The "Right" way to get the coupler off is to disassemble the whole truck (pull the springs out) and remove the frame around the bolster. Normally that ends with one or more springs being launched into oblivion. Ive started just drilling the coupler off the car with increasingly larger bits until the coupler mount is removed. I guess a dremel would also work. A more expensive alternative is to replace the trucks with Atlas ones, using the insert that Atlas provides with the separate sale trucks. If you use 3R trucks, modify as below.
Atlas cars are easy as well, remove the truck, dremel off the coupler and mounting arm, and screw on kadee. Since atlas uses their own coupler, the atlas brand ones are easier to fit (screw on. With Kadees, the holes wont line up with the kadee holes (naturally). the front one will go in and then use the box to drill the back one in the right spot. On hoppers and tank cars (things where the coupler goes into an end platform), just use the atlas couplers. With the atlas couplers, I replace the atlas springs with kadee springs, which are softer and make the couplers work a little easier.
Weaver cars are pre-drilled for Kadees. They just screw in. You need to cut the coupler arm off but keep the piece that goes between the truck and the car body in most cases.
Always check your work with the #812 height gauge.
