This is a model of a 50-foot flatcar. The trucks/couplers I suggest for this model are Micro-Trains #956 (Roller Bearing Trucks).
I print my models on a filament (FDM) printer. That's what they're designed to be printed on.
For weight and strength the car should be printed at 100% infill. Print this flat-side down and no supports will be needed.
