Many Managers believe replacing a trestle with a culvert will reduce maintenance and replacement cost, and figures seem to back this up, but it is necessairly true. I have replaced many culverts after 15 to 20 years due to corrosion from acid soils, and erosion from gravel washing through the pipe. Bituminous coating can aleviate this to some degree, but bituminous coating is expensive and heavy, adding to the material, transportation, and installation cost. Not to mention that it is awful stuff to work with. And, when a pipe does need replacing you must dig out and replace the fill as well. Granted much of the fill material can be re-used, but some is always contaminated or lost.
A further consideration is that a trestle repair can be carried out while still in service, where as a culvert repair (replace) requires closing the line. This does not matter if the line is shut down for winter, but weather and in steam work restrictions might preclude that. On the National Forest here, work in fish bearing streams is normally restricted to low water periods, which are usually Aug. and Sept.
BTW, I used an interesting new wood product from Boise Casade Corp. on my new house. It is a variation on the glue-lam process, made with thin layers of wood glued up in the vertical plane. This stuff is very strong, and is almose indistinguishable from solid wood. I don't think it would work for piles, but it would make a lower cost alternative to (if obtainable) solid wood stringers.