Relaisstellwerk

Relaisstellwerk is an electromechanical railway signaling interlocking system that uses electrical relays to implement routing and safety logic. It controls signals and point machines and ensures that a set of movements can be authorised only when it is safe, preventing conflicting routes.

In operation, an operator interacts with a control desk, often a lever frame or pushbutton panel. By selecting a route, the interlocking logic via relays evaluates the requested movements against the current track occupancy and other route requirements. If the route is safe, relays energize to position signals and switch points accordingly and to lock the chosen route in place. Track circuits provide train detection, so a route remains locked until it is cleared. If a train occupies a route or a conflicting movement is attempted, the interlocking blocks or releases the controls to maintain safety.

Key features describe the architecture: a cabinet or series of relay panels housing numerous relays, power supply circuits, input devices, and output circuits to signals and point machines. The design is inherently fail-safe; in the event of power loss or relay failure, signals default to danger to protect railway safety. The system often includes locking mechanisms to physically or electrically lock switches and ensure routes cannot be altered while occupied.

Today, Relaisstellwerke were widely deployed on mid-20th-century networks in German-speaking regions and have largely been superseded by electronic and computer-based interlockings. Some examples remain in service on older or rural lines, while others have been preserved in museums or heritage railways. The term also serves as a historical reference to this class of electromechanical signaling equipment.