block signal
One of the danger signals or safety signals which guide the movement of trains in a block system, often coupled with a switch so that opening or closing the switch also operates the signal.
block signal: the traffic light that protects railway sections
A block signal is a fixed wayside signal that controls train movement through a section of track called a block. It displays whether the block ahead is occupied, clear, or available at reduced speed. The signal sits at the entrance to its block and prevents a train from entering until the preceding train has cleared the block beyond, or until sufficient distance has been travelled to ensure safety. This principle, called block signalling, is the foundation of railway traffic management on main lines and busy branches.
Block signals work in tandem with track circuits or other detection methods. When a train occupies a block, a relay or modern electronic logic sets the signal to Stop (red or red and white). As the train clears the block, the signal clears progressively to Caution (yellow) or Clear (green), allowing the next train to proceed. Signals may display two aspects (Stop and Clear) on simple lines, or three or more aspects (Stop, Caution, Clear) where trains run at higher speeds and need earlier warning of changes ahead.
The mechanical coupling between signal and switch mentioned in common definitions refers to the interlocking mechanism. Where a block signal guards a junction or grade crossing, operating the signal lever also moves the points or crossing gates, ensuring that the route is set before the signal clears. This mechanical bond prevents operators from giving conflicting instructions: the signal cannot clear unless the switch is thrown and locked in the correct position.
Block signals are most commonly mounted on a post or bracket on the lineside, 6 to 8 feet above rail level. Early examples used semaphore arms or lower quadrant arms; modern installations use colour light signals with red, yellow, and green lamps. The signal receives its instruction from track circuits running under or alongside the rails, or from coded rail transmission systems that detect train presence and speed.
Failure modes in block signalling are critical. A signal that fails to clear (or clears when a train is in the block) creates a collision risk; one that fails to show Stop when it should creates a hazard. Modern block signals are therefore designed fail-safe: loss of power, breakage of wiring, or relay failure causes the signal to drop to Stop and lock. Maintenance crews check aspect visibility, lamp operation, and relay response on scheduled intervals and after any incident.