The runway is one-dimensional, but aircraft still move in 3d.  That means planes can be closer (in time) behind each other.  Trains must leave room enough to stop.  Planes can escape by diverting higher&#x2F;lower&#x2F;left&#x2F;right.  So planes stacked up on approach to a runway can safely be far closer together than trains on approach to a station.<p>Non-stopping trains are interesting, but such a solution would require the multiple parallel lines I mentioned above.  And even the non-stopping trains would need some amount of space between them for emergency stopping and speed changes.