He is looking for those "Adaptions", which attach to the road trailer .' The prototype was called a "RoadRailer", or something like that. I'm not sure that they are used by any railroads these days, at least not in Canada.
Aristo marketed them at one time, but I don't think they were very popular items. If he wants more of the end pieces, he must have some trailers that he thinks will work with them.
The Idea was that road trailers could be linked together to form a string, pulled on rails by a standard freight locomotive, without involving the extra weight and unloading facilities needed with flat cars or the newer "Intermodal" cars. (Piggy-Back cars).
It was the Railroads' marketing efforts to transport trailers long distances without the need of a driver for every truck, and avoiding heavier and heavier traffic on highways. And, of course regain some of the freight business lost to road transport (Trucks).
All was lost to the now VERY familiar, Containers, which now rule, on rail, or road transport. "Intermodal" transportation is the big game these days, nationally, and Internationally.
As far as the actual "Coupler". Any coupler can be used, even a draw bar, between modules. Standardizing on one make of coupler is best, but some people enjoy having certain cars, in certain trains, and only need same type couplers in those trains. Drawbars, can be used between cars if the price of couplers hurts one's budget.
I often send out "Care packages" of 3 pairs of Bachman, Aristo, USTrains, and Lionel trains couplers to friends, free of charge, just to brighten the day for Friends....!!
FJM