@John in California posted:I was thinking of going the other way and take an old Lionel boxcar that is an actual 3/16 scale and replacing the O gauge trucks with American Flyer S gauge trucks. The only problem is that the center of the S gauge truck will have to be mounted about a quarter inch towards the ends of the boxcar. This will require reworking the underside of the boxcar maybe adding a new bolster to mount the S gauge trucks.
Fender washers make good bolsters. You can vary the thickness as required. When the car is running, no one will see the underside.