Detectives with the Union County Prosecutor’s Office are investigating the death of a 17-year-old boy found dead Thursday night on train tracks in Plainfield, authorities said. The teenager, whose name was not released, was found about 8 p.m. on the tracks at West Second Street and Central Avenue, according to the prosecutor’s office. NJ Transit, Plainfield police and local fire officials were on the scene Thursday night, along with the prosecutor’s office. A spokesman for the prosecutor’s office said Friday no cause of death had been established and an investigation was ongoing. Anthony G. At…