block post

A block post is a signal box located at the boundary of a block section, the fundamental unit of railway signalling. Its job is to control the entry of trains into that section and to receive notification when a train occupies or clears it. The block post operator, or signaller, manages signals and interlocking equipment that prevent more than one train from entering the same block section simultaneously, which is the core principle protecting against collision.

Each block section runs between two block posts. When a train enters the section governed by a block post, the signaller sets the signals to "danger" (red) to prevent another train following behind. As the train moves through the section, the signaller receives an "all clear" indication from the block post at the far end, confirming the train has exited. Only then can the signaller clear the signals for the next train. This relay of information between block posts creates an interlocked chain of safety along the line.

Variants and signalling methods

Block posts may be equipped with different signalling systems. On lines using electric block working, instruments at the block post indicate the state of the section ahead: occupied, clear, or "line blocked". On mechanical railways, pegs or tokens physically removed and replaced signal occupation. Modern signalling centres control multiple block sections remotely, but the principle of the block post remains: it is the point where human decision and mechanical safety apparatus meet to regulate train movement.

The name "block post" comes directly from the block system itself, in which the line is divided into blocks. A post is simply the physical location and structure, housing the signalbox, levers, instruments, and the signaller. On busy lines, block posts are spaced closer together (sometimes 1 to 2 miles apart) to increase line capacity. On freight branches or lightly used routes, they may be several miles apart.

The block post is distinct from a junction signal box, which controls switches and crossings, and from crossing keeper's huts on level crossings. Its sole function is to enforce block section discipline. It remains a critical node in railway operation, and on lines still using traditional signalling, the block post and its signaller are essential to safe train working.