A 4,000-mph magnetically levitated train could allow you to have lunch in Manhattan and still get to London in time for the theater, despite the 5-hour time difference. It's not impossible: Norway has studied neutrally buoyant tunnels (concluding that they're feasible, though expensive), and Shanghai is running maglev trains to its airport. But supersonic speeds require another critical step: eliminating the air -- and therefore air friction -- from the train's path. A vacuum would also save the tunnel from the destructive effects of a sonic boom, which, unchecked, could potentially rip the tunnel apart. Popular Science