I too have resisted posting on this subject figuring that someone would post the same observations I am going to make.
Historically speaking most trestle fires have been started by the trains that use the trestle. Cinders, spraks from brakeshoes, hotboxes and improperly used torpedos and fusees are the most common causes.
Typically we only started fires on the way up the hill in the morning and we ran a fire patrol behind the train. After the train departed from Cumbres to Osier the helper engine (if one had been used) would depart for Chama. After the helper departed the fire patrol would turn and head back to Chama looking for any fires that might have been missed on the way up. Occasionally we held the helper at Cumbres until both trains had arrived at Osier and been inspected in case it was necessary to use the helper to replace one of the road engines. In those cases the fire patrol went down before the helper. We never had a fire patrol wait at Cumbres to follow the afternoon train to Chama. As far as I can recall fires between the rails or right next to them were the result of leaking or over full ash pans. If the ash pan is not closed properly or the ash pan is rusted through, or the ash pan screens are not in place hot cinders can fall out of the pan even when the engine is working under a light load or drifting. Locomotives returning to Chama from Antonito frequently had very full ash pans.
As regards fire prevention a couple of thoughts come to mind. Railroads commonly covered the bridge ties with sheet metal to keep cinders from getting to the ties. This method is relatively low tech, inexpensive, and only needs to be done once. Another option is to spray the bridge timbers with a fire retardant like "Phoschek" every year when the right of way weed spray is done.
In my opinion safety systems need to be simple and inexpensive to be effective. Complicated and expensive systems dont work, the principle is known as KISS.
John Bush