block section
In railway signalling, a section of track on which, in normal circumstances, only one train is allowed at one time.
block section: one train, one track segment, one signal rule
A block section is a length of railway track that the signalling system treats as a single unit for the purpose of train separation and safety. Only one train may occupy a block section at any given time under normal operating conditions. The section is delimited by signals, typically a home signal at its start and a distant signal marking its approach; when a train enters, the signal behind it clears to red to prevent a following train from entering until the first has cleared the section entirely.
Block sections vary in length depending on line speed, track geometry, and traffic intensity. On high-speed routes, sections may extend 3 km or more to permit safe braking distances; on commuter or suburban lines with frequent stops, sections may be 500 metres or less. The length is calculated to ensure that a train traveling at line speed can come to a complete stop before striking a stopped train at the far end of the section ahead.
The principle underlying block sections is simple: if you know where trains are to the nearest section, and sections are long enough for safe stopping, then trains cannot collide. Traditional block working relies on a signaller or lineside operator to know which sections are occupied and to set signals accordingly. Modern automatic block signalling (ABS) uses track circuits, axle counters, or other detection systems to sense train presence and change signals automatically without human intervention.
When a track circuit fails or an axle counter misreads, the system must assume the section is occupied and hold signals at red until the fault is cleared. This causes delays but prevents the risk of unsafe signal clearance. A train may be unable to clear a section if it breaks down; in this case, the signaller or driver must take manual action to protect the train and route traffic around it, typically by placing detonators on the rail and stopping following trains.
Block sections are fundamental to railway safety and remain in use on nearly all signalled lines worldwide, whether controlled by traditional manual signal boxes or modern traffic management systems. The term 'block' comes from the analogy of dividing the track into separate blocks, each occupiable by only one train at a time, creating a fail-safe separation mechanism.