Block System
2 of the 7 encyclopedias on this shelf carry an entry for Block System. Both are reproduced below, so you can see where they agree and where they differ.
Collier's New Encyclopedia (1921)
in railroad parlance, the division of a railroad into a certain number of telegraphic districts, the distance between which is determined by the amount of traffic, each block station having signaling instruments by which the signal man can communicate with the operator on each side of him. When a train enters any block a semaphore signal is lowered, and no train is allowed to follow until the one in front has reached the end of the block, when raised and at the same time the signal is lowered for the block ahead, etc. The block systems used in Europe and in the United States generally employ mechanical and electrical devices for lowering and raising the signal.
Aiton's Encyclopedia (1910)
a method of preventing collisions on railways. According to this plan a line of railway is divided into sections guarded at each end by signal towers or semaphores operated by local officials in constant communication by telegraph. No train is allowed to enter a section until the display of a wooden arm on the signal tower indicates that the section is clear. In this manner no two trains are permitted to be on the same section of track. If signals are displayed properly and obeyed, collisions are impossible. See Railroad 2026 Editor's Note: The principle here — never two trains in one section — remains the foundation of railway signalling worldwide. Only the machinery changed: track circuits, then automatic signals, and now digital systems in which trains report their own positions continuously and a computer keeps a moving safety envelope around each one. (Ed: BR 2026-06-03)