The thing about wood trestles is that there are bazillions of them around the United States alone (or at least there were). I don't just mean the big 1000 foot spans like you guys have modeled in the images above. I mean all the smaller ones. Every gully, ever small stream,
every 50 foot gap in the ground had to have a bridge. So there were bazillions of them. (Plus or minus 1 gillion) Each railroad built their own bridges, and they did those over a century long span. So the PRR may have used a different design for it's smaller trestles than the B&O used. And the PRR might have used 1 design on a line in 1900 and used a different design on a line they laid in 1940.
So there are many many many different designs of wood trestles. But there are only 2
main kids of wood bridges. Short and tall. Short wood bridges use the brute strength of the wood to support the heavy train. Wood is a lot like concrete. It doesn't compress, but it will snap in half. So short bridges (shorter than the height of trees) often use
big wood poles like this that support all the weight. Sometimes those are round,
sometimes they are square. Those lateral braces aren't supporting anything. They are just keeping the support poles from moving around. This is a total brute force design, saying the train isn't heavy enough to cause 6 of these poles to buckle. On some smaller trestles you might only see 4 supports. The basic concept is still big columns supporting the weight. But you'd get lots of variations on the lateral supports that hold the columns in place. These variations (see the two examples linked earlier in this paragraph) give each bridge with the same design a very different look.
On larger trestles brute force isn't enough. You can't just put a tree trunk under the tracks and have it support the train as it will snap like a twig because the added length gives it more lateral bend. (Think about a wood 1 foot ruler and a wood yard stick. Same wood. Same thickness. Same width. But you can bend the yard stick quick a bit, while the 1 foot ruler doesn't bend. The added length gives the wood more flexibility. And that's a big huge negative when you are trying to support weight. You want rigid, not flexible.)
So larger trestles go with a much more intricate design. Basically it's just one set of trusses stacked upon another. They can be built with much smaller wood boards. You don't need tree trunks for every plank. You can use 8x8's and 4x4's because the truss design spreads the load on each portion of the track across the entire bridge. These had a more standardized design because the concept requires both horizontal support and lateral bracing. But they also are more expensive to build and unique to the terrain they are on, so each one was individually designed. So many of the large wood trestles look similar, but they are all somewhat different. (Spacing of the lateral struts, angle of them, do they make X's between each set of vertical columns or do they alternate like /\/\/\/\/\ , or are the lateral struts horizontal? etc etc etc)
For a more modern comparison, look at the Twin Towers in NYC. The vertical support for those builds was the big steel columns around the outside of the building. Each floor of the building locked the outer vertical supports in place laterally, so they couldn't bend and all their support was directed straight up. The fire after the planes crashed caused the floors to collapse, leaving the vertical supports without their lateral bracing. The result was the 20 floors worth of weight on top of the crash site being too much weight for the outer support columns, which caused them to buckle. Once they started buckling, there was no way to stop it. Same thing with a wood bridge. If you can keep the vertical supports from bending, they will support enormous amounts of weight. But if they start to buckle at all, the battle is lost.
What I'm getting at is that there is no 'correct' design of a wood train trestle. There are totally wrong designs (You don't see a lot of wooden suspension bridges, and when you do i
t's mostly just the decking that is wood) but no single correct design. You don't have to model an existing bridge exactly. As long as you follow some of the basic design elements of wood bridges, it will turn out realistic....even if the design elements you use wouldn't scale up to make a usable bridge in real life.
In the 2 photos above this post, the bottom one is very realistic in the way it's designed. The top one is trying to mix the short bridge design with tall bridge height. If it was a real bridge those long support columns that are not linked together would buckle under the weight. But that said.....once it's painted and the scenery is added, it'll still look awesome. 98 out of 100 people looking at it will have no idea it wouldn't support a real train, and 99 out of 100 people wouldn't care. For the 1 out of 100 that made a comment about it, you can hand him a pile of bass wood sticks and tell him to do it better or shut up. If he does do it better, then great! You get a free new bridge. If he just continues to run his mouth, drop something heavy on his foot so he whines about something else for a while.
It's a model railroad. It doesn't have to be 100% accurate.
I have a steel bridge on my layout that's totally structurally unsound. (Look at the right bridge on the link) It's a steel span in an S curve shape. I backed myself into a corner with the track layout and had to do it to make it work. In real life that bridge would be totally straight because in real life an S curved bridge like that without support columns under the apex of each curve would buckle like a dog's hind leg. You will also notice that my wood trestle is a short design, and absolutely doesn't need the lateral supports I put on, but I thought they looked cool. So it's completely unrealistic, but it looks cool on the layout.
This is all different than the newer steel bridges. Where they all have minor differences, they are mostly (I said mostly. I know there are some totally different designs) small variations on designs like
this... this... and
this....
