When you consider the cost-prohibitive replacement value of a Lobato-size trestle, the fact that the track deck is highly flammable, the fact that a deck fire can destroy much of the non-flammable ironwork, and the fact that they run trains that routinely set fires to ties and vegetation, it seems to me that the hazard is so obvious that it is hard to believe nothing was done about it before the Lobato fire.
But now that the lesson has been learned, I think strong measures to prevent a fire on a repaired Lobato bridge would be most prudent. The flammable track deck has got to go. I understand the desire for historical authenticity, but most everybody is willing to compromise that goal somewhat in the interest of practicality.
Concrete ties would be one solution. Steel ties would be another solution. The wood deck walkway should replaced with a steel walkway. If wood ties are used, they will be special ties, and their quality should be exceptional. These dimensionally uniform, high quality wood ties, with proper pre-drilling, could be rather easily protected by a well-fitting sheet metal cover.
Aluminum would be one option, but I would prefer something around 1/16th inch thick, which may be more cost effective if made of steel and then hot dip galvanized for excellent, long-term rust protection. They would cover the entire top and sides of the ties. For the tie ends, the covers would just overhang about 3” at each end. If the metal is relatively thick, the 3” overhang will hold up well without getting bent. And the overhang would also be supported by the side flanges.
A sheet metal shop could produce the tie covers. Guessing they might cost $150 each, the covers for 175 ties of the Lobato deck would cost $26,250. When you add this to the cost of the ties, it may be more cost-effective to use concrete or steel ties.
The smaller all-timber trestles are fire hazards too, but they are much less costly to replace, and much harder to protect with metal, since any part of the trestle can catch fire. So the issue is with Lobato, Cascade, and any others in that league if there are any.