A man about 50 years old was struck and killed by a train on the MBTA commuter rail tracks in Worcester Thursday morning, officials said. Train P512, which had left Union Station in Worcester at about 7:30 a.m., struck the man near the 800 block of Franklin Street, said MBTA spokesman Joe Pesaturo. MBTA Transit Police are investigating. The accident caused inbound commuter rail delays as police investigated, Pesaturo said. All stops between Southborough and South Station were affected by the delays. Passengers on the affected train were transported to another train for the remainder of their commute.Jasper Craven can be reached at firstname.lastname@example.org. Follow him on Twitter @Jasper_Craven.