Ten people have been killed and up to 200 injured, some 40 of them critically, in a crash involving three trains in Los Angeles.
About 300 firefighters are involved in the rescue effort
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).
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.
He identified the driver as Juan Manuel Alvarez, 26, of Compton.
"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.
Following the collision, 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.
"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, in light rain, searching for survivors in smoking and overturned carriages.
Helicopters and sniffer dogs were brought in to help to help find people trapped in the wreckage.
Around 200 people were treated on colour-coded plastic mats set up close to the crash site to act as temporary triage areas.
Officials said 100 people were taken to hospital and described it as a "very difficult morning".
Did you witness the crash? Send us your comments and experiences using the form on the right.
Very early this morning, when I was waking up I could hear sirens. The emergency vehicles were on the way to the crash. I heard sirens, all different kinds of them, for what seemed like an hour. A massive response had been launched. It is typical to hear sirens in LA but never have I heard so many of them, for so long, and so loud. It was quite chilling, and I hope to never hear that again.
Jim Bornzin, Los Angeles
I cross these tracks every morning to get to work in Glendale. Like most rail crossings, the tracks have barriers that come down when a train is nearing. The crash took place about a 1/2 mile from my work. It is in an area near some homes, but mostly is in a business area. I'm sorry for the loss of life.
Michelle, Glendale, USA
It's really not a very good sign that accidents like this happen still today. But thank God that there are only few people killed by this crash which is a little wonder although it were three trains.
Korbinian Ober, Munich, Germany
I commute to work in Los Angeles on the Metrolink trains every day of the week. To think that someone would deliberately put a car on the tracks - knowing that it will cost lives - is unbelievable. My heart goes out to all the lives and families affected by such a careless act.
Lynn, Ontario, California, USA
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.