I use a small flathead jeweler's screwdriver to pry off the end doors on the Red Caboose/Intermountain autoracks.
I cut up 6 1/2" x 1/2" paper strips and glue my cheap bulk N scale cars (from wehonest.net) onto them using Elmer's Clear Glue (regular white Elmer's glue will work just as well). Carefully center the cars when gluing them onto the paper strip. Then after drying, place the paper strips into each level of the car and pop the end doors back in.
The total weight will be about equal to that of the MTL 89' auto rack, so if you arrange your autorack consist according to car weight, you can place these towards the front of the train along with the MTLs.