A teenager was killed subway surfing in Brooklyn Monday morning, police said. The teen struck his head on an overhang at the Fourth Ave./9th St. stop in Park Slope about 10:10 a.m. then fell to the tracks and was run over by the G train, cops said. He died at the scene.  His age was not immediately clear as cops worked to identify him. The NYPD and city officials have been pushing to alert young people to the dangers of subway surfing after a string of deadly mishaps, many fueled by social media. Gardiner Anderson for New York Daily NewsPolice investigate after a teenager was killed while subway surfing at the 4th Avenue – 9th Street station in Brooklyn on Monday morning. (Gardiner Anderson for New York Daily News)