A Massachusetts transportation official says "operator error" is the focus of an investigation into what caused a six-car passenger train to leave a suburban Boston station without a driver.
None of the approximately 50 passengers on the Massachusetts Bay Transportation Authority train was injured.
The train traveled for about nine minutes - and through four stations - before MBTA personnel were able to stop it by cutting power to the electrified third rail.
Secretary of Transportation Stephanie Pollack said Thursday that before the train left the Braintree station, the operator got off the train to execute a procedure that allows it to travel if there is a signal problem.
Pollack said an investigation is under way to determine if the driver failed to properly secured brakes before exiting the train.