This is a model of a 40' flatcar. The trucks/couplers I used for this model are Micro-Trains #003 02 021 (Bettendorf 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 the car flat-side down and no supports will be needed.