Interlocking

An interlocking is the arrangement of switches, signals, and detection devices at a junction or crossing that is engineered so that conflicting train movements are physically impossible to authorise. The principle dates from the 1850s and is the foundation of safe railway operation: a route is "set" by aligning the switches in a particular configuration and clearing the signals that protect it, and the interlocking logic — mechanical, relay, or computer-based — refuses to clear any signal whose route would conflict with another already cleared.

Mechanical interlockings, where rods, levers, and tappet bars in a frame physically lock the lever combinations, were the dominant technology from the 1870s to the 1950s and survive on heritage and minor lines to this day. Relay interlockings replaced them mid-century with banks of vital relays whose contacts wired the same logic in electrical form, capable of controlling far larger junctions remotely. Modern computer-based interlockings (CBI) implement the same logic in software running on dual-vital processors, with the same safety properties proved by formal verification against a mathematical model.

In North American operating practice, an interlocking is often called a "control point" and carries a specific name (CP Aberdeen, CP East Wallingford) used over the dispatcher's radio. In British and Commonwealth usage, the equivalent term is "signal box" or, for the more recent computerised installations, "signalling centre" — Manchester ROC or Three Bridges ROC each control hundreds of miles of route from a single building.

For railfans, an interlocking is also a place — the physical location where you'll see signals coming off one after another in a route-setting sequence, the switches snapping over, and the trains threading the resulting paths. Lining them up on a stack of cameras at a busy interlocking with multiple routes is one of the more rewarding genres of trackside photography.