BOSTON (AP) — Massachusetts Gov. Charlie Baker says the investigation into a runaway passenger train has pointed to several violations of safety procedures by the train's operator.

The driverless Red Line train left a station Thursday morning and rumbled through four stations before transit officials stopped it by cutting power to the rails. No passengers were injured.

Transit officials say the train began to move after the operator stepped out to engage a "bypass" switch to override a signal problem.

Secretary of Transportation Stephanie Pollack said Friday the train's emergency brake wasn't engaged. She said investigators are looking into reports that the train's throttle had been tied back with some kind of cord.

Pollack called the driver's actions "irresponsible."

The driver has been placed on paid administrative leave pending further investigation.