These are the stl files I used to build my n-scale model of the Burlington Iowa swing span bridge. The fixed span has a bottom, 2 sides, 2 top ends, and 4 top middles to print. I added 0.015 brass wire for the top cross bracing. This isn't a super detailed print but makes for a quick 9" double track through truss. After sanding, all parts were glued together with CA. The bottom girders are supported on 0.05" styrene to line up with the shore though your installation may vary. I cut Chooch piers to fit. The top of the bridge can lift off an on to access track as it is supported separately from the bottom.
The swingspan is quite a bit more work. The swingspan pier is made from 17 bents on each end, 24 sides, a center, and upstream and downstream end pier. I printed the sides 6 at one time and just place them on my print bed close enough that they formed one part. The bents were glued to one side spaced a scale 4" apart. The other side was then glued to form the wood structure. I added strip styrene for the walk way and hand rail. The swingspan is made of two bottom pieces that I fit into the the base. I glued the pieces together to make the entire structure able to actually swings should someone want to figure out the rail locking and precision alignment. The upper structure includes a left and right side, two center supports, and center overlay along with two different sizes of middle top supports. I fabricated the top ends from and extra center. I also added 0.015 brass wire cross bracing and 0.01x0.02 center bracing.
The end girders are micro engineering 40' plate girders modified. There are two kinds of telephone wire towers. I scratch built supports from styrene and brass. The telephone arms are the top of Model Power poles. I added Gold Medal Models brass ladders and scratch build walkways and hand rails. My entire bridge is just setting in place and can be entirely removed in pieces if necessary which is nice. I still need to build a center walkway between the tracks and scratch build a control house on the side of the swingspan. While I wanted a lighter bridge with visible x-braced girders and rivets, I'm pretty pleased with how it turned out. Good luck and take your time.