Crews were called to Cedar Avenue near Golden State Boulevard after 1 am.
They found an RV engulfed flames, with the fire starting to spread to a nearby building.
Crews were able to keep the fire contained to the RV, with minimal damage to the building.
They searched the area, but no one was around.
"There was a lot of equipment inside that could have burned if nobody protected this fire initially, but I'm glad that we were here to get it," said Fresno Fire Battalion Chief Tony Escobedo.
Crews say the biggest challenge was accessing fire hydrants across the train tracks.
They had to make sure there were no passing trains as they put out the flames.
The cause of the fire is under investigation.