This video of a toy maglev train is a great illustration of how the technology works.
Watch the whole thing...there's a nice bit at the end with tracks mounted vertically on buildings. (via cyn-c)
Popular Science published an article five years ago on the possibility of a trans-Atlantic maglev train that would travel in an airless underwater tunnel at 4,000 MPH and make the trip from New York to London in an hour.
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.
