The crash near the town of Chatsworth caused a fire and at least one commuter train carriages and seven cars from the freight train to derail.
Firefighters battled a fire under part of the wreckage and pulled people from the commuter carriage, which was partly torn open after toppling onto its side.
As many as 350 people were on the Metrolink train. The Sheriffâ€™s Department confirmed three people had been killed.
According to other reports, 4 people were killed.
One of the largest medical facilities in the area, Northridge Hospital Medical Center, was told to prepare for the arrival of injured passengers.
Firefighters pulled passengers out a rear door and down a ladder from the toppled commuter car, which had been separated from the rest of the train by several feet. Crumpled and charred freight cars were strewn across the tracks. Dazed and injured passengers sat on the ground and milled about on both sides of the tracks.
Any news from people on the ground.