The MBTA is pointing the finger at the operator of the Green Line trolley that derailed Wednesday morning in Newton, saying a preliminary investigation found that the driver did not have the go-ahead to proceed just before the derailment.

The Green Line train heading inbound on the D Branch was pulling out of Riverside Station in Newton just after 6 a.m. Wednesday when the second car in the two-car train derailed at a track switch, the T said. The only passenger on board was not injured and the T dispatched dozens of shuttle buses to replace rush hour service for commuters.

The T said its initial investigation into the derailment found no problems with the train, track infrastructure or the signal system. Instead, investigators are "focused on the operation of the train as it departed Riverside Station," the T said.

"The preliminary investigation shows the train's operator did not have the signal system's authorization to proceed. By not allowing the track switch to be properly aligned, the second car of the train came off the rails," the agency said in a statement. "The train's operator, who was hired in March of this year, has been removed from service while investigators complete their work."

The derailed car was put back onto the tracks at 11 a.m. and the T said that regular service on the Green Line's D Branch would resume shortly after 11:45 a.m.

"I want to apologize to the Green Line customers whose commutes were disrupted this morning," MBTA General Manager Steve Poftak said in a statement. "We will complete the formal investigation as soon as possible and take corrective action if needed. We can and we must do better."

The Green Line's derailment Wednesday comes as riders on the Red Line are still coping with suboptimal service since a Red Line train derailed June 11 in Dorchester, wiping out signal infrastructure as it careened off the track. That incident, which came just days after another Green Line derailment blamed on operator error, spurred the T to form a review panel to review the agency's safety practices and derailments.

The safety panel includes former U.S. Secretary of Transportation Ray LaHood, former acting administrator of the Federal Transit Administration Carolyn Flowers and former New York City Transit President Carmen Bianco.

T officials have not announced the cause of the Red Line derailment, but this week said that delays stemming from the crash will extend until sometime in October. The T initially hoped to restore normal service by Labor Day.