Anti-subway-surfing barriers have been installed on a string of No. 7 cars to try to stop kids from attempting the deadly maneuver — but some straphangers aren’t convinced they’ll thwart the young scofflaws. The padded barriers have been placed toward the top between train cars and are designed to prevent would-be surfers from climbing on the roof a car — but critics say the contraptions could actually give the rogue teens a boost to their destination, or at least do little to stop them. “It