BOSTON (CBS) — The MBTA said they are investigating a “serious incident” in which a Red Line train left Braintree Station Thursday morning without an operator.

The 6:08 a.m. inbound train did not make any stops, but traveled past four stops. The train was brought to a halt just past North Quincy Station, when crews powered down the third rail.

At that point, T employees boarded the train, driving it to the JFK/UMass stop to allow passengers to exit. The train was taken out of service and examined.

No passengers were injured, but a 51-year-old train operator who has been with the MBTA for more than 28 years was brushed by the train and suffered minor injuries.

The MBTA said their investigation includes looking into an initial report that a safety device in the cab of the train may have been tampered with. They are currently interviewing witnesses and the train operator.

“Passenger safety is the highest priority for the MBTA and this highly troubling incident is under investigation by Transit Police detectives,” said MBTA General Manager Frank DePaola in a release.

A person with knowledge of the trains told WBZ-TV’s Lauren Leamanczyk this was a “very dangerous situation for passengers.”

Initial indications are that a safety device inside the train’s cab may have been tampered with.