Boxcars.
My first design is with integral roof and roofwalk.The floor is a separate part, attached to the body by 4 bits of sprue.Take care to not damage the stirrups when separating them.
The floor has 4 notches in it near the ends for the truss rods.I used a loop made from EZ Line, about 5 inches long.
The handbrake assembly passes through the platform and lower support at one end (arrowed).
Weight can be added onto the top of the floor before final assembly.
Assembled:
I was not entirely happy with the test prints.The layering (stepping) was quite evident on the roof.This effect will of course vary with print orientation.
By painting and sanding I was able to achieve a smooth finish.To avoid this I've designed a version without a roof.Plastic card (styrene sheet) is used instead for the roof.You will need a piece 27x89 mm, scored down the middle.This also works out cheaper.The floor is integral with the body in this version, with holes for the truss rods.The roofwalk is attached to the body by some bits of sprue.
I have not test printed this design yet, but in theory it should work ok.