The accident occurred around 5:45 (1345 GMT) near the city of Oxnard, about 75 miles (120 kilometers) from Los Angeles, according to news reports.
A local fire official told CNN that authorities have detained the driver of the truck, who fled the accident scene.
Television images showed that several train cars had careened off the track, and the truck engulfed in flames.
The Metrolink commuter train was carrying scores of passengers toward Los Angeles at the start of the morning rush hour.