Luis,
First, I have to agree with Vic about Plastruct. I like styrene for most everything. With all the shapes, and sheet roof,and siding from Evergreen, there aren't many projects that can't be built with it. There are a number of adhesives that work well with styrene, my favorite being MEK.
In the case of rolling stock, and buildings, that have a coat of paint that usually covers the wood grain(not to mention that these things are built with "finished"lumber), styrene works just as well as wood. Trestles, however are different in that they are usually built with rough cut timbers, and the wood grain is more visible. They are not painted, but treated, so the grain isn't hidden.
I wouldn't use anything but basswood for trestles. The practice of staining the lumber before assembly is the best, however, the use of an oil base stain could make it a little difficult using some glues. The bond might not be as good. Thinned laquer, or alcohol with some coloring agent would be preferred to oil base stain.
If you decide to go with the plastruct, you could spray the whole assembly with something like Floquil "foundation", and then drybrush or "wash" weather it.
Pete