Hi, Not only are they deck trusses, but they are also metal ones. That makes things complicated as that means using sections., which is likely to be more expensive.
The first photo is of a ‘lattice’ deck truss, and alas is not clearer than that as it is a bit small and possibly out of focus.
The second (ex road bridge) has a lattice also this time is it made from angle iron, Evergreen make plastic sections as do Plastruct the latter being ABS (special cement needed) the former being thinner and made from styrene which can be glued with normal ‘plastic’ cement.
For outdoor use, the best choice may well be to use aluminium angle and (lots and lots of ) stainless steel bolts, or possibly spend more and get an Eaglewings bridge made for you - expensive yes, and I think they are welded together, but it should last! Theey are powder coated I believe (I live in the UK by the way NOT the USA) for better preservation, bridges are a pain to paint, especially metal ones - really the best way is a spray gun, and a garage on a calm day, at least aluminium does not need painting! I have one that came as a kit from Germany, and is very good - it is also about 4 foot long - its a through deck girder bridge but the basic principles are the same - just longer pieces of angle between the top and bottom of the bridge!
All bridges will take a prodigious amount of angle by the way, and that may well suprise you - they ain’t cheap!
Depending on the length you may also be able to hide (under the track) a couple of square section tube section in steel to give the whole bridge some strength, if so rustproof it before painting it. Mine came from a DIY chain store (Home Depot?), they (in the UK) are about 3 foot long.
Wooden bridges are easier but no less complicated, I have made a 4 foot long Howe Truss and that took a lot of material, it has welding rod for the tension rods and the rest is PVC solid foam (Sintra in the USA) plastic strips and lots of glue as well.
Finally , there is a book by Kalmbach, on bridges: my old copy does not have much on specifically deck trusses, but there is a lot of general bridge knowledge for both metal or wood bridges.