This is an instructable for building a Plate Girder Railroad Bridge in N-Scale or (1:160). The 3D model files can be found on Thingiverse.
Step 1: Print the Parts
The parts for this model can be printed with any 3D Printer but there is detail (rivets) that will not show up unless you have a printer with a 250 micron nozzle. There is also an option to print a solid road bed with a walkway and hand rail. The walkway and Handrail do require a 250 micron nozzle.
Note that I model in full scale dimensions so you will need to downsize the STL files to N-Scale (1:160 or 0.62).
The parts shown above are for a standard bridge across which bridge track would be laid. You need two end pieces, two sides, and a top and a bottom. The bottom and top are different in that the bottom has rivets showing.
Also seen in the picture are assembly jigs. Pins and cork board could suffice.
Step 2: Insert the Bottom and the Two Sides Into the Larger Jig
This will be a tight fit! I put the bottom piece in with the rivet detail facing up but if the bottom of your bridge is to be visible they should probably face down.
Step 3: Insert the Smaller Jig
Note that if you look at the underside of the parts you will see a nice flush surface! Super glue the sides into place when all is neat.
Step 4: Insert and Glue the Ends.
The inner jig can be used to help position the end pieces. Make sure it does not get glued!
Step 5: The Partly Assembled Bridge...
...should probably get a coat of paint now while you can get the inside!
Here it is after an initial base cote. The top can now be glued into place. Weathering is next but I have not figured that out yet!