Coil springs are molded as an integral part of the coupler. Patent Pending Spring Retainers. Easy installation and operation. Fits most HO scale cars with built-in draft gear boxes (coupler pockets). Compatible with all magnetically operated knucklecouplers. These couplers are center shank-medium. Operate with a standard metal "glad hand" and under track magnet. CHOKING HAZARD - CONTAINS SMALL PARTS - NOT FOR CHILDREN UNDER 3