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How do railway signals work?

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Quick answer

Railway signals keep trains apart by splitting the line into blocks that only one train may occupy. Track circuits or axle counters detect each train, and the signal guarding an occupied block shows red while earlier signals show yellow. Interlocking ensures points are set correctly before any signal can turn green.

Railway signals work by dividing the track into sections called blocks and allowing only one train into each block at a time. Detectors sense where trains are, and an interlocking system sets each signal's colour so drivers know whether the track ahead is clear, restricted or occupied.

Step by step

  1. A train entering a block is detected by a track circuit or by axle counters at the block boundary.
  2. The detection equipment marks the block as occupied and sets the signal at its entrance to red.
  3. Signals further back show double yellow and single yellow, warning following drivers to slow down in time.
  4. Before clearing a route, the interlocking checks that points are set, locked, and no conflicting signal is clear.
  5. When the train leaves the block, detection confirms it is empty and the signals step back toward green.

Key things to understand

  1. The line is divided into blocks, each protected by a signal at its entrance.
  2. Track circuits or axle counters detect whether a train is occupying a block.
  3. An occupied block turns the signal behind it red, and the signals further back show yellow warnings.
  4. Interlocking logic stops a signal clearing unless the points are set and locked for a safe route.
  5. Train protection systems can apply the brakes automatically if a driver passes a red signal or overspeeds.

Labelled diagram

  1. Occupied block

    A train sits in this section, detected by a track circuit or axle counter.

  2. Red signal

    Guards the entrance to the occupied block and tells drivers to stop.

  3. Single yellow

    Warns the driver to be prepared to stop at the next red signal.

  4. Double yellow

    Gives an earlier warning on fast lines where longer braking distances are needed.

  5. Green signal

    Shows the blocks ahead are clear, so the train may run at line speed.

Block signalling keeps trains safely apart

Real-world examples

  • Busy Indian main lines use automatic block signalling so several trains can follow each other closely between stations.
  • Indian Railways is installing Kavach train protection to stop trains automatically before a red signal.
  • On Britain's network, the Train Protection and Warning System can trigger emergency braking at certain red signals.
  • Many metro lines use communications-based train control, where computers track train positions continuously instead of fixed blocks.

Common mistake

✗ Myth: A signalman manually watches every train and switches each signal by hand.

✓ Fact: On most modern lines, train detection and interlocking set signals automatically, and controllers supervise whole routes from a central control room. Human operators still set routes and handle failures, but safety-critical checks are built into the equipment.

People also ask

How do train signals know when a train is coming?
Track circuits or axle counters tell them. A track circuit passes current through the rails, and a train's wheels short it, dropping a relay that sets the signal to red. Axle counters at each end of a block count wheels in and out, declaring the block clear only when counts match.
What does a double yellow signal mean?
A double yellow aspect is an advance warning in four-aspect signalling. It tells the driver the next signal is showing a single yellow, so the train should be ready to slow. It gives fast trains extra distance to brake before eventually meeting a red signal.
What happens if a train passes a red signal?
Railways call this a signal passed at danger, or SPAD, and treat it as a serious safety incident. Train protection systems such as Kavach, TPWS or ETCS are designed to apply the brakes automatically, and overlap distances beyond many signals give extra room to stop.
What do the signal colours mean?
In typical multiple-aspect colour-light signalling, red means stop, single yellow means be prepared to stop at the next signal, double yellow warns the next signal may show single yellow, and green means proceed at line speed.
How does a track circuit detect a train?
A small current flows down one rail, through a relay and back along the other rail. When a train enters, its steel wheels and axles connect the rails, short-circuiting the relay so it drops and turns the signal red.
Why do trains need signals when they run on fixed tracks?
A heavy train needs hundreds of metres, often more than a kilometre, to stop from high speed, far beyond what a driver can see ahead. Signals give warning early enough for the train to stop before reaching an obstruction.
What is Kavach on Indian Railways?
Kavach is India's indigenous automatic train protection system. It shares signal and position data between trackside and onboard equipment and applies the brakes if a driver fails to respond to a stop signal or exceeds speed limits.

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