About 300 firefighters are involved in the rescue effortTen people have been killed and up to 200 injured, some 40 of them critically, in a crash involving three trains in Los Angeles.
A Metrolink commuter train struck a car, derailed and collided with another commuter train. One of the trains then ploughed into a parked freight train.
The accident happened at the height of the morning rush hour in the Los Angeles suburb of Glendale.
Police have arrested a man and are expected to bring charges.
Hundreds of firefighters searched the scene for survivors.
"For me this is the worst train accident I have ever seen," said LA Fire Department chief Captain Rex Vilaubi.
The two double-decker Metrolink commuter trains were travelling near Union Station in downtown LA - one heading to Burbank and another from Moorpark - when the crash happened at 0600 local time (1400 GMT).
The train tilted, everyone was screaming and I held onto a pole for dear life
Police Chief Randy Adams announced the arrest of a "deranged" man who had been driving the car, and said he would be charged with homicide.
"I think his intent... was to take his own life but changed his mind prior to the train actually striking this vehicle," he told reporters.
One of the train carriages caught fire and another ended up on its side.
One of the trains also hit a stationary Union Pacific freight train, said to be carrying gravel, pushing it off the tracks.
Investigation
"I heard a noise. It got louder and louder," passenger Diane Brady, 56, of Simi Valley, was quoted by the Associated Press news agency as saying.
"And next thing I knew the train tilted, everyone was screaming and I held onto a pole for dear life. I held on for what seemed like a week and a half, it seemed. It was a complete nightmare."
Television pictures showed some of the 300 firefighters at the scene searching for survivors in smoking and overturned carriages.
Helicopters and sniffer dogs were brought in to help, but it was unclear whether people were still trapped in the wreckage.
Plastic mats were set up close to the crash site to act as temporary triage areas. Many of the injured arrived on stretchers.
LOS ANGELES TRAIN CRASH
The crash involved two double-decker Metrolink commuter trains (1&2) one heading south into LA Union Station, the other heading out. It is unclear which is which at this stage. Police said one of the commuter trains hit a car parked on the tracks and then "side-swiped" the other train. Commuter train 2 struck a stationary freight train (3) which was parked overnight on a spare track.
It looks like the man responsible for this may be eligible for the death penalty. Kind of ironic since he intended to commit suicide, but changed his mind at the last minute.