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.