I'm late to the party, but I wanted to chime in and say that Marx did sell 6" tin cars with Tab & Slot couplers in electric train sets. The electric versions have rivets holding the coupler to the cars, whereas the (later) windup cars with T&S couplers have a simple twist or punch that keeps them in place. The windup couplers will slide in and out. The electric versions with the rivets will back up better around curves. Since the windup couplers slide in & out, they will bunch up when pushed and the carbodies will rub against each other, which can make them more prone to derail when backing through curves.
For that reason, I would strongly suggest that you look for 6" tin cars with riveted T&S couplers, plus they should have metal wheels as well.
