Chemical fire retardants are available. At the Roaring Camp we spray our wooden trestles on both the standard gauge and narrow gauge lines with a chemical retardant called Phos-chek. It is easy to apply and we have it done at the same time as we do the weed spraying along the right of way.
My personal recommendation would be that the timbers be treated with Phos-chek and them covered with the sheet metal deck I suggested earlier.
In my experience sophisticated and technically complex systems are not effective. How many of you live in houses with burglar alarm systems which have in fact been turned off for a variety of reasons including but not limited to false alarms, forgotten dis-arm codes, dead batteries, and broken sensors?
K I S S
John Bush