A Big Apple straphanger was repeatedly slammed in the head with a metal pipe at a Queens subway station — the latest example of the recent outburst of violence in the city’s troubled transit system. The latest underground assault took place shortly before 1 a.m. Saturday at the Queensboro Plaza station when a thug walked up to the 31-year-old victim and struck him in the head “multiple times” in an unprovoked attack, according to police. The bearded attacker then fled, leaving the victim unconscious. He was taken to New York Presbyterian/Weill Cornell Medical Center to be treated for several cuts...