I love wood! I love wooden boats, houses, and trestles. But wood is expensive to purchase and maintain, not to mention a fire hazzard. That is the reason my rural house has concrete board siding, and differed maintenance is the reason my wooden boat is no longer sea worthy. This does not mean that appearence needs to be forsaken to utility though. The siding is made to look like board and batton, and you have to look closely to tell that it is not. Likewise, a trestle need not be made of wood to look like a real trestle. On the branch line from La Grande to Joseph Oregon there is a four or five span trestle that I drove by many times, and thought that it was a neat looking wooden one. Then one day I was riding by, rather than driving, and saw that it was in fact made of steel. Other than being painted dark brown, no attempt was made to disguise it. By selecting the right components a steel trestle could be made to look like wood. While the initial cost would be as great as wood, possibly even higher, the life span would be almost indefinant and maintenance reduced to little more than inspection and painting.
I hate to sound like a heratic, but historic preservation is expensive and does not pay well at all. Sometimes it is better to put your money into things that count, like locomotives, and settle for something less in places that are less obvious. But not in shiney steel culverts.