State Transportation Secretary Stephanie Pollack acknowledged this afternoon that the train's driver left the train to prep it to deal with signal problems that have plagued the Braintree branch of the Red Line all this week.
But at an afternoon press conference, Pollack added that "operator error" is the primary focus of an MBTA investigation into the incident - in which a train traveled from Braintree to North Quincy without a driver.
Pollack and MBTA COO Jeff Gonneville said that due to the signal problems, the driver asked for and got permission to put the train into "bypass" mode by toggling a switch at the front of the train that would let it proceed through red signals. Before leaving his cabin, they said, he should have applied both the train's main brakes and a separate hand brake - which is located about three feet from the control panel at which he normally sits.
"While the operator was not on board the train, the train left the station," Pollack said. She called what happened "an unacceptable breach of our responsibility to keep our riders safe."
Pollack did not say how the train could have started moving without anybody at the controls. She said part of the investigation will include looking at the cord at least one passenger on the train reported seeing around the train controller.
She added that the reason the train traveled as far as it did - for nine minutes - was actually a safety measure: Supervisors at the T's operations control center kept the power on so that trains in front of the ghost train could get out of the way first.
Once they were out of the way, the operations control center shut power between JFK/UMass and North Quincy and the train slowly came to a stop.
She said the T did not let the roughly 50 passengers on the train know what was going on until T workers actually showed up at the train because the system has no way to communicate with passengers on trains.
She continued another focus of the investigation will be whether the train stayed at or below the 25-mph limit for trains in "bypass" mode.