Some good samaritans spotted the flames late Friday night, and helped rescue a homeless woman who was living under the bridge.
Bellingham fire marshal Jason Napier says police found small propane cylinders on the side of the creek where the woman was rescued.
He says the woman may have been using the propane to stay warm.
The bridge was once part of the Bellingham Bay & British Columbia railroad.
It was made of wood beams coated in creosote, which is a tar-like substance that can be hard to extinguish when it catches on fire.
It took fire crews a total of 10 hours to finally put out one of the creosote-covered beams.
Police are looking for the woman, and the fire is still under investigation.