The Collaborative International Dictionary of English v.0.48:

Block system \Block system\ (Railroads)
   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.]