\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}.