Interlocking
An interlocking is the signaling apparatus, relay-based or computer-based, that ensures signals, points (switches) and track sections are set in a safe, non-conflicting configuration before a route is cleared for a train. It is the core safety logic that prevents conflicting train movements.
How it works
When a route is requested, the interlocking checks that every point is set and locked in the correct position, that the track sections are unoccupied, and that no conflicting route is set. Only then does it clear the protecting signal. While the route is in use, the interlocking locks the points so they cannot be moved under a train.
Older installations use relay logic; modern computer-based interlockings (CBI) implement the same rules in fail-safe software, typically with redundant processors, and increasingly communicate with field elements over IP networks using standardized interfaces such as EULYNX.
Why it matters for security
The interlocking is the single point where signaling safety is enforced. Computer-based interlockings and their maintenance, diagnostic and object-controller networks introduce IP connectivity into what was once a physically isolated system. Unauthorized access or manipulation of interlocking communications is among the highest-consequence scenarios in rail OT security.
Related solution
Protecting interlocking systems at the core of rail safety
Securing route control, signaling logic and switch operations in safety-critical environments.
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