A woman was fatally struck by an inbound MBTA commuter train on the Fitchburg line in Concord Friday night, the MBTA said. The woman, whose identity was not released, was believed to be about 18 to 22 years old, Transit Police said. The accident occurred at 8:08 p.m. and the woman died at the scene, said T spokesman Joe Pesaturo. About 20 people were aboard the train, he added. The accident was under investigation by Transit Police, and no further details were immediately available.