A Blue Line train was evacuated just after midnight on Sunday when a small fire started on the tracks inside of the Aquarium Station tunnel, according to an MBTA spokeswoman. The fire was started by a spark emitted from an insulator on the rail and was very minor, MBTA spokeswoman Kelly Smith said. When firefighters responded to the scene, power on the tracks was turned off while they extinguished the smoldering rail, according to Boston Fire Department spokesman Steve MacDonald. Nearly 200 passengers were evacuated from the train, he said. The MBTA provided shuttle busses for Blue Line passengers though the end of service and trains operated on a normal Sunday schedule in the morning, Smith said.