A Salem teenager was struck and killed by an MBTA commuter rail train near the west end of Ocean Avenue in Salem shortly after 10 p.m. Thursday, officials said. Dominic DeMartino, 18, was hit by a train headed for Newburyport and was pronounced dead at Salem Hospital shortly afterward, said Carrie Kimball Monahan, a spokeswoman for the Essex district attorney’s office. Officials said the young man was with an unknown number of friends at the time, but all signs indicate that the death was an accident. It was the second fatality on the commuter lines in the past three days. A 21-year-old Roslindale man was struck and killed by an inbound commuter rail train in Hyde Park Wednesday night, MBTA spokesman Joe Pesaturo saidMelissa Werthmann can be reached at firstname.lastname@example.org.