Rescuers were quickly on the scene of the crash
At least 18 people have died and 135 have been hurt in a head-on collision between a passenger train and a freight train near Los Angeles.
The crash derailed both trains and forced the engine of the passenger train back into one of its coaches.
Fire-fighters put out a blaze under one of the carriages and have been cutting through metal to rescue trapped people.
The exact cause of the crash - the worst in the US for 15 years - is not yet known.
The crash happened at 1632 local time (2332 GMT) on Friday, at a time when the passenger train was carrying 222 people, most of them commuters.
The Metrolink passenger train was travelling from Los Angeles to Moorpark, north-west of the city, a spokeswoman said.
It collided with a Union Pacific freight train on a curving stretch of track in Chatsworth, in the San Fernando Valley area of Los Angeles county.
"That's probably the most serious trainwreck to occur here in a very long time," Los Angeles Mayor Antonio Villaraigosa said, according to the AFP news agency.
The front coach of the Metrolink passenger train derailed and was crushed by the engine after the collision. Two other coaches of the train remained upright.
Aerial images of the crash scene showed teams of rescuers using ladders to reach injured people inside the mangled front coach.
A number of casualties were treated near the scene, while some were airlifted to hospital.
There are fears that the death toll will rise further.
Los Angeles City Fire Captain John Virant described the scene as "total destruction".
"It was chaos. The injuries are crushing," he was quoted as saying by the Los Angeles Times newspaper.
Capt Virant said firefighters were trying to cut through train carriages to rescue the injured.
"They are in there removing dead bodies that are lying on top of survivors," he said.
The Union Pacific freight train was badly damaged in the accident. Officials say two people - the engineer and the conductor - were on board the train.
"I heard a loud crash and I saw black smoke... some people were mangled pretty bad," Phil Thiele, one of the passengers in the front coach, told the Los Angeles Times.
He said he tried to help one man who was pinned between seats: "I tried my damnedest to get him out but I just couldn't."
Mr Thiele also said one woman appeared to have a serious head injury.