A 19-year-old man was transported to Tufts Medical Center yesterday after he fell onto the Red Line tracks in the southbound pit at Park Street Station, according to an MBTA spokeswoman. MBTA officials were notified of the fall around 6:10 p.m. The man, who was identified as Richard Morrow, was conscious and alert on the way to the hospital. He suffered a head injury, according to MBTA spokeswoman Lydia Rivera. Service resumed in both directions after about five minutes, she said.