About 50 other people were wounded in the incident.
Dr Stephanie Hall, the chief medical officer at Los Angeles County-USC Medical Centre, said three people were in a critical condition - two females and a male.
Rescue crews put out a fire caused by the collision and worked to free trapped survivors as heavy equipment was brought in to move the carriages.
Julio Pedraza, a witness, said that the crash "made a terrible sound, like a bomb, a huge noise".
Denise Tyrrell, a spokeswoman for the Metrolink rail system, said that the train left Union Station in Los Angeles and was travelling northwest to Moorpark.
She said that there would normally be about 350 people on board the train in rush hour.
We don't know if we hit another train or another train hit us," Tyrrell said.
The worst disaster in Metrolink's history was in January 2005, in suburban Glendale, when a man parked a sport utility vehicle doused in petrol on the tracks. A Metrolink train struck the vehicle and derailed, striking another Metrolink train travelling the other way, killing 11 people and injuring about 180 others.
Juan Alvarez was convicted this year of murder for causing the crash.