This is a 1/32 scale US 50 feet Boxcar run by Union Pacific. With Daniel Norée's EMD SW1500 (https://www.myminifactory.com/object/openrailway-emd-sw1500-1-32-locomotive-12438) and my 8 axle heavy duty flatcar (https://www.myminifactory.com/object/26071), it will be part of a Nasa train (check picture below)
Check the exploded view to know what goes where.
Lay the three parts of the bottom, sides and roof flat on a table. First dry test the assembly of each group. To align the parts I designed tenon and mortise joints. The slots in contact with the bed could be printed too small and you may need to widen them a bit with a cutter.
After gluing the 3 parts, let them dry with a heavy weight on top to avoid warping.
!! Before gluing the right part of the side frame, insert the door into the rails of the middle part !!
Or you'll have a hard time installing it later... with the risk of breaking it or bending it permanently.
Then glue the sides to the bottom following this order : left side - end - right side - end.
If you want to paint the inside of the car, now is the moment . It's indeed better to glue the roof on the body as it helps keeping the sides flat.
Then you can glue the brake assembly to one of the end parts. Before gluing the grab irons and the ladders, bore the holes with a 1.2 mm bit.
The knuckle couplers and their gearbox (18) are the same as the ones for my "8 axles flat car" and can be found here : https://www.myminifactory.com/object/26467 . Use 3x20mm screws to fix them to the bottom.
The trucks (19) can be found here :https://www.myminifactory.com/object/26027. Fasten them to the bottom using parts 16 and 17 with a 3x30 mm screw.