I own a Standard Gauge RichArt Milwaukee Road passenger set that includes a tri-color Bipolar and five tri-color passenger cars. All came from RichArt with box couplers installed. The box couplers are plastic and are identical to the box couplers found on many if not most G scale trains. I believe you can easily find these box couplers for sale online and at any train shop that sells G scale trains and parts. Based on their appearance and what RichArt did, it should be relatively simple to replace most Standard Gauge metal couplers with these plastic box couplers.
Simply cutoff the end of a Standard Gauge coupler, drill a hole in what remains, then bolt the above box coupler to the metal end of what remains of the old coupler using the existing hole.
Bob
