No longer in business, JnJ Trains used to sell cast resin passenger car kits as well as "brass car sides". Said sides were designed to be mounted to pre-existing passenger car bodies (either American Ltd passenger car "core" kits, or mass-manufactured cars from the likes of Kato, Con-Cor and Bachmann). JnJ's catalog was extensive, covering numerous different roads and car styles.
I'm told that the cast resin cars were mostly knock-offs of old brass models and that the quality was not particularly high (thick walls, warping, etc).