Brutal video obtained by The Post reveals how a gruesome, fatal subway surfing incident in Lower Manhattan unfolded. The clip shows the 14-year-old boy and his 18-year-old pal on the top of the J train as it zoomed along in Lower Manhattan Friday – before one of them is struck in the head by a beam. The footage shows both falling to the rails and ground below. The 14-year-old boy was killed and the 18-year-old was gravely injured, cops said. The younger teen died as a result of injuries