Railway track circuits are fundamental safety systems that detect the presence or absence of a train within a specific section of track. They work by creating an electrical circuit that uses the rails themselves.
The Basic Principle
Each section of the railway forms part of an electric circuit, which runs a current from one rail to the other through a relay. This simple concept is key to detecting trains. We use rails to link a power source at one end of a section with a relay at the other end.
When the track section is empty, the electrical current flows uninterrupted from the power source, along one rail, through the relay, and back along the other rail to complete the circuit. This energized relay indicates that the section is clear.
Components of a Track Circuit
Understanding the components helps clarify their function:
- Power Source: Provides the electrical current for the circuit. This could be a DC or AC source, depending on the type of track circuit.
- Rails: The two running rails of the track serve as the conductors for the electrical circuit.
- Relay: An electromagnetic switch located at one end of the section. It is central to the detection process.
How it Detects a Train
The magic happens when a train enters the section. The metal wheels and axles of the train act like a short circuit, providing a much lower resistance path for the electricity to flow across the rails.
- Instead of the current flowing through the high-resistance path of the relay, it now largely flows through the train's wheels and axles.
- This diverts the current away from the relay.
- With insufficient current flowing through it, the relay de-energizes (drops).
The Relay's Role: Signalling Safety
The state of the relay (energized or de-energized) is directly linked to safety systems like railway signals.
| Track Section State | Current Flow Through Relay | Relay State | Signalling System Indication |
|---|---|---|---|
| Unoccupied | Yes (sufficient) | Energized (Up) | Clear / Safe |
| Occupied | No (insufficient) | De-energized (Down) | Occupied / Unsafe |
When the relay is energized (track clear), the associated signal can display a 'clear' or 'proceed' aspect. When the relay is de-energized (track occupied), the signal is automatically set to 'stop' or a restrictive aspect, preventing other trains from entering the occupied section.
Practical Applications and Benefits
Track circuits are a cornerstone of railway safety and operations:
- Signal Control: Automatically controlling signals based on track occupancy.
- Level Crossing Protection: Activating warning systems (lights, barriers) as a train approaches.
- Interlocking: Ensuring safe movements at junctions by preventing conflicting routes from being set simultaneously.
- Automatic Train Protection (ATP): Providing vital occupancy information to onboard safety systems.
Their passive nature (relying on the train to short-circuit the track) makes them a reliable detection method. Various types of track circuits exist, including DC, AC, and audio frequency types, each with specific advantages for different railway environments and operational needs.
