The Los Angeles Times and local TV station KTLA both said at least 30 people were hurt, and KTLA said one may have died.
The collision just before 6am PST (1400 GMT), north west of Los Angeles, overturned at least three double-decker Metrolink rail cars, according to live video on CNN.
Metrolink spokesman Scott Johnson told CNN passengers were being transported to area hospitals and family and friends were being notified.
‘Multi-casualty’
The Ventura County Fire Department said the incident was being regarded as “multi-casualty”, but it could not confirm the number of deaths.
The US National Transportation Safety Board tweeted that it was gathering information about the incident.
Train cars were lying on their side at the scene, where tarps were laid out to treat the injured as firefighters and paramedics attended to crash victims.