A system by which the track is divided into short sections, as of three or four miles, and trains are so run by the guidance of electric, or combined electric and pneumatic, signals that no train enters a section or block until the preceding train has left it, as in absolute blocking, or that a train may be allowed to follow another into a block as long as it proceeds with excessive caution, as in permissive blocking.  [Webster 1913 Suppl.]