Russ Fischer, Marvin Casias and I were on the crew that day. I recall they were intending to have 2 different explosions: one was to be a big smoke cloud, the other was to be an incindiary (sp?) fireball. They two explosions were to be processed together in the editing room). We told the director that one one of us MUST be on the set when this event was to happen (which why all 3 of us were there that day). They decided to set them off while everyone on the production crew was on lunch break. So we were sitting in at picnic tables enjoying typical movie craft services chow when we heard a muffled "boooom". To the west we saw a big black cloud which disipated fairly quickly. We didn't like the looks of this, so we choaked down our lunch and started heading towards Marvin's truck.
Another "booooom". Another big cloud of smoke. This one doesn't disipate. More smoke. As we reach Marvin's truck, one of the movie crew drives up and asks if we have a water truck. We look at him and say, you want us to call the fire dept? "Oh.... no, a water truck will be fine. Marvin runs in the shop call the fire dept. and we blast off up the road. We encounter Willie Nelson's tour bus coming at us on the narrow dirt road. Marvin turns on his headlights and we force old Willie off into the weeds. By the time we got there (less than 15 minutes after the second boom) the entire trestle in engulfed in flames from one end to the other.
We later heard that after the incindiary blast went off a small fire started at the end of the trestle (the blast was supposed to be sufficiently away from the trestle to avoid that). As the crew started in with fire extinguishers (the only fire fighting equipment they had), the director told them to stop and let it burn a bit while they filmed the flames. The typical spring San Luis Valley Zephyrs came up and away the fire went.