The teen was at the time with a group of friends on a northbound R train at the 74th Street-Roosevelt Ave station in Jackson Heights about 10.25am Monday in Queens, according to police and law-enforcement and MTA sources.He was trying to climb to the top of the train and fell, landing on the tracks, cops and sources said, NY Post reported.“We know he was subway surfing on the outside of the car,” a transit worker at the station told The Post. “Usually when you’re subway surfing, you have to get off the train before the tunnel. He must’ve miss that point.”The boy’s left limb was detached “from the elbow down,” and, “they found his arm under the train,” the worker said. He seemed overwhelmed by shock, the employee said of the kid.“He was just looking around like nothing happened. He didn’t cry or nothing,” said the worker, who wouldn’t give her name. The boy was taken to Bellevue Hospital and is in stable condition, police said.As of noon, northbound M and R trains were bypassing 74th St...