A few years ago, Micro-Trains released a series of Ringling Brothers Circus cars in N-scale. This included several flat cars and old time circus wagons. I wanted to run a circus train with these cars on the flat cars but since there wasn't anything to hold the wagons onto the flat cars, they would fall off on curves. I really did not want to glue them down because I wanted to be able to remove them to simulate delivery to the circus location. After some research on the web, I saw that these older circus wagons did not appear to be chained down but appeared (from the available B&W pictures) to just have their wheels blocked and maybe had something holding them down at their axles.  It was really hard to tell. So since I did not want to glue the circus wagons to their flat cars, I came up with a simple 3d printed "H" shaped fixture that would be glued to the car (The fixture could also be made out of styrene).  The fixture would then allow me to put the wagons onto the car and take them off but would hold the wagons in place throughout the layout.