Someone who works nearby shot video from a rooftop, showing the fire burning on the bridge at Paul Avenue and Carr Street. Then, moments later, the flames jumped to the hillside below.
The grass fire posed a problem for the fire crew because trains were still running and firefighters didn't want to be in their path.
Once that was controlled, crews then focused on snuffing out the flames on the bridge.
There were two trains that passed under the blazing bridge, so fire crews worked fast to get that under control.
"Once those boards start going and they burn, they'll fall down onto the tracks and they pose a serious derailment hazard," SFFD Battalion Chief Charles Crane said. "So, it's a good thing that somebody called it in."
Crews had to pull chunks of the wooden footbridge apart.
The fire also damaged pipes under the bridge, one of which carries raw sewage. Right now it's spilling on the tracks below
Crews have blocked off the footbridge. It's so badly damaged it will have to be torn down, and rebuilt.
Workers from the Department of Public Works are also there to deal with the leaking sewage pipe under the bridge.
Caltrain did have to delay trains for a while Saturday afternoon. But the trains eventually returned to their regular schedule.