I did a truss bridge for a friend's HO layout way back...most likely sometime in the early 1990's as MTH and Weaver were just getting into the swing of six-axle road diesels at the time). It was made of craft sticks. Y' know, like popsicle sticks. I only wish I knew where the sole photos of it I took are, but they were taken before I got my first digital camera so it's anyone's guess where the prints are. I took a couple of hours to hack together a digital drawing of how it looked:
How I designed it is still fresh in my mind (save for the exact makeup of the splice plates as they were fabricated from parallel craft sticks). How to make a 3' or so bridge out of what are essentially popsicle sticks? Make I-beams out of them. I sawed off the rounded ends of many sticks to square them off, then assembled them into I-beams, which were then joined end-to end with 1" bits of stick glued across the web of said girders as splice plates on both sides. Each of these joints were further reinforced by drilling and inserting common stick pins through them, securing them against movement with glue,then clipping off the excess. Once I had two of these beams of the requesite length I moved on to the actual deck.
Since this was for HO scale. I fabricated crossbeams of the same sawed-off sticks, butted perpendicular to the side I-beams, I didn't cap these with flanges, but had I been doing this for O-scale, I definitely would have. Running from crossbeam to crossbeam were stringers cut to fit each span and spaced to lie under the HO rails. Now the deck looked something like a ladder inside a wider ladder. For bracing I cut additional sticks with 45-degree angles to atttach to the underside of the ladder, turning it into a truss. With the decking done, I turned to the superstructure.
The superstructure was as one would expect for a curved upper chord. All structural members here were built up of the same squared-off craft sticks asembled into I-beams, laid out on their sides with the vertical beams oriented with the web facing the ends of the structure and the flanges parallel, extending reaching a little more than a half-inch farther in order to join the flanges of both top and bottom I-beams. All splice plates in the superstructure were also drilled and pinned, although as mentioned above, I'm not certain of exactly how I arranged them.
So how strong was it? I didn't have any HO locomotives to place within it, (the bridge certainly dwarfs them) but with the completed structure supported by VHS cassette cases at each end. I sat three O-gauge diesels atop it, a MTH Dash 8 and C30-7, and a Weaver SD40-2. About 15 pounds of locomotives, and there was no measurable deflection in the deck. The bridge also survived falling about four feet when one of the free-standing tables it spanned was accidentally bumped. One of the superstructure beams broke loose from the end of the deck owing to the lack of splice plates there--I took the bridge home and added them as pictured. I'm pretty sure if I were building a similar-length structure out of the same materials today for O scale, I could build it strong enough to safely carry a scale Big Boy or Allegheny.