The crash was reported shortly after 7 a.m. this morning, said Fire Department spokesman Brian Humphrey.
Circumstances of the crash were not immediately clear. Television reports showed one mangled car and another damaged vehicle alongside tracks on which a Metrolink train was stopped.
Humphrey said four people in the cars were injured and two were taken to hospitals in critical condition, including the infant.
At least six passengers aboard the train were being examined for possible injuries, he said.
The train remained upright and on the tracks.
Phone calls to Metrolink representatives were not immediately returned.
The Southern California train agency has been plagued in recent years by major crashes. Last year, 25 people were killed when a Metrolink train collided with a freight train. Investigators believe the Metrolink engineer ran a red light seconds after he was text messaging on his cell phone.
In 2005, 11 people died and about 180 were injured when a man who later claimed he was suicidal parked his sport utility vehicle on the tracks in suburban Glendale. The driver of the SUV was convicted of murder.