Overview
An Interlocking is the central safety logic in railway signalling.
Its purpose is straightforward:
Prevent conflicting train movements and ensure that every authorised route is safe.
Interlocking ensures:
- No two trains enter conflicting paths
- Points/turnouts are correctly set and locked
- Signals display aspects that match a safe route
- Any failure results in the most restrictive condition
It is the decision-making core of every station, junction, and yard.
Why is it called “Interlocking”?
The term comes from early mechanical lever systems where:
- signal levers
- point levers
- locking bars
…were physically locked together so that an unsafe combination could not be pulled.
Modern interlockings apply the same principle through logic instead of machinery.
Core Functions of an Interlocking
1. Route Setting
When a route is requested, the interlocking:
- checks track clearance
- commands points/turnouts
- locks the points
- clears the corresponding signal only if everything is safe
2. Conflict Prevention
Preventing conflicting train paths is the core duty.
For example, two converging lines cannot be simultaneously set towards the same junction point.
3. Fail-Safe Operation
Interlockings follow strict fail-safe principles:
any failure → safest state.
Examples:
- Power loss → signals go to danger
- Broken wire → relay drops
- Processor error → system goes restrictive
4. Infrastructure Monitoring
Interlocking supervises:
- track circuits
- axle counters
- points detection
- signal status
- level crossings
- interfaces to ATP or Train Protection Systems
Types of Interlocking (Globally Used)
1. Mechanical Interlocking
- Uses levers, rods, and mechanical locking
- Still found in some heritage railways and rural regions (e.g., UK)
2. Relay Interlocking
- Uses electrical relays
- Known for robustness and long life
- Widely used from mid-20th century to present
3. Electronic Interlocking (EI)
- Microprocessor-based safety logic
- Redundant hardware architecture
- Application logic stored in safe-coded software
4. Computer-Based Interlocking (CBI)
A broader term for software-controlled and digitally networked interlockings.
These systems offer:
- modular design
- remote diagnostics
- flexible scalability
5. Distributed / Virtual Interlocking
Used in advanced systems such as:
- Moving block
- ETCS Level 3 concepts
- High-automation metro lines
These may not have a single “interlocking box” but distributed safety modules.
Interlocking Inputs and Outputs
Inputs
- Track circuits
- Axle counters
- Point detection contacts
- Signal lamp proving
- Level crossing interlocks
- Train protection feedback
Outputs
- Signal aspect controls
- Point movement commands
- Level crossing activation
- Interfaces to Traffic Management Systems
Safety Principles
1. Route Locking
Prevents other conflicting routes from being set.
2. Approach Locking
Once a train is approaching, the route cannot be cancelled until conditions are safe.
3. Route Release
Automated or manual release after the train clears the route.
4. Flank Protection
Ensures adjacent tracks are protected from side collisions.
5. Fail-Safe Logic
If in doubt → treat as unsafe.
Where Interlocking Is Required
Any location involving:
- points or turnouts
- junctions and crossovers
- terminal stations
- yards and depots
- passing loops on single lines
Even simple crossovers require interlocking.
Country Variations (Brief and Neutral)
United Kingdom
- Significant presence of relay and electronic interlockings
- Mechanical frames still found in rural and heritage areas
- Interfaces with UK-specific systems like TPWS and AWS
Europe (EU)
- Strong adoption of EN 50126/50128/50129 safety standards
- Growing use of ETCS integration with interlockings
United States
- Mix of relay and digital interlockings
- Regulated by FRA and AREMA standards
- Large, complex freight networks
India
- Large-scale migration from relays to electronic interlocking
- High-density corridors use automated route setting
Japan
- Highly automated systems
- Strong reliability and redundancy requirements
- Integration with automatic train control systems
Common Questions
Is an interlocking the same as a signalling system?
No.
Interlocking is a component inside the signalling system.
It provides safety logic, not traffic management.
Can trains operate without an interlocking?
Yes, on simple lines without conflicting paths —
e.g., token systems on single lines with no points.
But any location with turnouts needs an interlocking.