block system

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A system for controlling and safeguarding the flow of railway trains in which track is divided into blocks, each controlled by automatic signals.

from the GNU version of the Collaborative International Dictionary of English.

• (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.