A total of 28 passengers were taken to hospitals, four with critical injuries, according to the Oxnard Fire Department.
Glenn Frisbie was driving to work and sitting at an intersection about a block away when the train struck the truck.
"I saw a bright flash, a big fireball and flames, flames going pretty high," he said.
Authorities were questioning the truck driver, whom they said left the truck on the tracks and was found several miles away after the crash.
For reason that are not yet clear, his truck was sitting on the tracks at a marked crossing around 5:44 a.m. as the train approached, fire officials said.
The Metrolink train carrying 51 passengers was heading from Ventura County to Los Angeles.
The stretch of track 65 miles from Los Angeles was straight, and that allowed the conductor to see the truck on the tracks and begin braking, Oxnard Fire Battalion Chief Sergio Martinez said.
The train would have been accelerating out of the Oxnard station at about 55 mph, Metrolink spokesman Scott Johnson said.
With braking, he estimated it would have hit the truck at between 40 mph and 55 mph, though that was not an official calculation.
Little was left of the truck except scorched and mangled wreckage, with some debris in a nearby intersection and some close to the tracks.
"When the crews arrived on scene, it was in flames, the vehicle, and it was pretty much cut in half," Martinez said.
The National Transportation Safety Board and the Federal Railroad Administration were sending investigators.
Television news helicopters showed several fire trucks surrounding the tracks running parallel to a road and a field. One toppled car laid with one end on the tracks and another on the street.
The locomotive, which was pushing the train from the back, was upright.
Metrolink trains have been involved in two major disasters in the past decade, among other accidents.
Twenty-five people were killed on Sept. 12, 2008, when a Metrolink commuter train struck a Union Pacific freight train head-on in the San Fernando Valley community of Chatsworth. More than 100 people were hurt in one of the worst railroad accidents in U.S. history.
Federal investigators later concluded that the Metrolink engineer had been texting moments before the crash and ran a red light.
In 2005, 11 people were killed and about 180 were injured when a man who later claimed he was suicidal parked his SUV on tracks in suburban Glendale and fled before an oncoming Metrolink train struck it and derailed, hitting a second Metrolink train.
A firefighter climbs into the wreckage of a Metrolink passenger train that derailed Tuesday near Los Angeles after colliding with an abandoned truck. (AP Photo/Mark J. Terrill)
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