block system
On railways, a system by which the track is divided into sections of three or four miles, and trains are run by the guidance of electric signals so that no train enters a section or block before the preceding train has left it.
block system: train safety through track division
The block system divides a railway line into fixed sections, typically three to four miles long, and uses signals to ensure only one train occupies any section at a time. Each section, or block, is protected by signals at its entrance; a train cannot proceed into a block until the signal clears, which happens only when the preceding train has fully exited. This prevents head-on collisions and rear-end impacts by making separation mandatory and automatic.
The system operates through track circuits: electrical circuits completed through the rails themselves. When a train occupies a block, its wheels and axles bridge the rails and alter the electrical state of that section. This change triggers the signal at the block entrance to show "danger" or "stop," holding back any following train. As the train advances and leaves the block, the circuit resets and the signal clears for the next train to enter.
Variations and implementation
Two main variants exist: absolute block system, where only one train may occupy a block under any circumstance, and permissive block system, which allows a second train to enter at reduced speed if the line ahead is clear. Permissive blocks are used on heavily-trafficked lines where absolute blocks would severely limit capacity. Modern railways also employ automatic train protection (ATP) systems that supplement or replace manual signal observation, though the block principle remains the foundation of train separation.
Block length varies with line speed and traffic density. Fast main lines use longer blocks; congested suburban routes use shorter ones. A block too long creates dangerous spacing; too short and signal equipment becomes costly and complex. The signal itself, traditionally a mechanical arm or colored light, must be positioned so drivers can observe it with sufficient distance to stop.
The block system is fundamental to all railway safety regulations and has been in continuous use since the mid-19th century. It survives in modern signaling because it provides a simple, verifiable rule: no two trains in one block. Failures in block system maintenance, such as fouled track circuits or sighted but ignored signals, remain leading causes of railway accidents. The system's effectiveness depends entirely on honest operation and proper maintenance of both signals and track detection equipment.