The tank car was parked on a spur off a main rail line along Interstate 215 in Riverside County, about 92 kilometres east of Los Angeles.
However, the tanker was filled in Texas 50 days ago, and one possibility was that a stabilizing chemical in the tanker had broken down, said Mark Scoville with the California Department of Forestry and Fire Protection Riverside County. Drones equipped with infrared sensors were monitoring the temperature, which appeared to be dropping but remained too high for firefighters to safely approach, authorities said at an afternoon news conference.
When conditions are safe, firefighters planned to move away a dozen tanker cars surrounding the chemical container and then eventually move it away to a safer location, officials said.