A woman in her 30s was struck and killed by a northbound Orange Line train at Stony Brook Station in Jamaica Plain Wednesday afternoon, authorities said. The unidentified woman was on the inbound track toward the south end of the station when she was struck, MBTA Transit Police said, citing preliminary reports from officers on the scene. Transit Police responded to the scene at about 2:45 p.m., along with Boston police, fire, and rescue workers, the statement said. Transit Police are investigating, the T said. Shuttle buses replaced train service between the Ruggles and Forest Hills stations, the MBTA said.