The Assembly file is primarily for display, but it is also shows how the kit parts are assembled.
The parts are modeled in 1/2-inch = 1-ft scale. The numbers in the file names are the number of parts of that type to be incorporated in the assembly.
I got the basic model, “wagon bogie” by Ivan Liu as an Autodesk 123D Design file from the Autodesk gallery. I used 123D Design and Meshmixer to modify the parts for 3D printing. All parts except the axels are flat on one side for printing without supports. The brake mechanism in the center is simplified as a silhouette of the multiple moving parts of the original model. I used the wheels from Makerbot's Big Boy http://www.thingiverse.com/thing:259005, because they had better resolution than the ones from Liu's model.