A teenage boy was struck and killed by a train while walking along a SEPTA commuter rail line track in Middletown on Friday.   Police identify the youth as 15-year-old Chris Mongillo of West Highland Avenue in Middletown.   He was a student at Neshaminy High School.   Investigators say the boy left a note at home.  The Bucks County Coroner is calling the death a suicide.   It happened on the same SEPTA track where another Neshaminy High School student was struck and killed by a train on New Year's Day.