interlocking tower
A building or room that houses controls for switches.
interlocking tower: the signal box that prevents train collisions
An interlocking tower is a fixed structure containing mechanical or electrical apparatus that controls the position of railway switches and signals across a defined section of track. Its core function is to prevent conflicting train movements by making it physically impossible to set two incompatible routes at the same time. Historically, these were dedicated buildings; modern installations may be housed in compact rooms or control centres remote from the trackside.
The mechanical heart of a traditional interlocking tower is a frame of levers, each controlling a switch or signal. Mechanical locking bars physically prevent a lever from being moved if another incompatible lever has already been pulled. A lever for signal A cannot be operated if the lever controlling the switch that signal guards is set to route a train toward an occupied section. This hardwired logic prevented human error long before electronics existed.
Most interlocking towers were built between the 1870s and 1960s and housed 20 to 200 individual levers depending on track complexity. The typical layout placed the lever frame where signallers could see the trackwork they controlled, often at junctions or busy stations. As railways electrified, relay-based interlocking replaced mechanical frames, then solid-state systems replaced relays. Modern computerised interlocking dispenses with the tower entirely, though the term persists.
Common failure modes in mechanical towers include stick or wear in the lever frame under heavy use, and occasional entrapment of the mechanical lock bars by debris or ice. Signallers operated these towers round the clock in shifts; the role demanded focus and knowledge of every route and signal in the area. The interlocking tower was the absolute safeguard: if the locking frame could not be forced into an unsafe position, no collision could result from signaller error.
The term "interlocking" refers directly to the interlocking bars that prevent unsafe combinations of lever positions. Historic interlocking towers remain on many heritage railways and some active lines, still functioning as designed. Their mechanical simplicity and absolute reliability made them an elegant solution to the problem of coordinating movements across complex track layouts in an era before telecommunications and computers.