Authorities said the suspect, 46-year-old James Alfred Burnett, is a registered sex offender.
Surveillance photos show him on the train's platform at around 2 p.m. Thursday.
The suspect tried to drag a young girl off the train, according to the sheriff's department. Bystanders grabbed him, and the young girl fled.
"He had grabbed her in a bear hug, and he was dragging her off. And this group of passengers, could be up to 10 people, actually prevented this from occurring. They tackled him and they brought him to the ground," said Steve Whitmore of the Los Angeles County Sheriff's Department.
The suspect fled on another train but was captured at the next station. Sheriff's investigators hope the unknown victim will contact them.