Relaisstellwerken

Relaisstellwerken, or relay interlockings, are a form of railway signaling installation in which electromechanical relays implement the interlocking logic that controls signals and point machines. They were widely used on German-speaking railways from the late 19th century through the mid- to late 20th century, forming the backbone of many mainline signaling systems before electronic interlockings became common.

A Relaisstellwerk typically comprises a control panel used by the signaller, a cabinet housing the relay logic, and a wiring network that connects to track circuits, signals, and point machines. The interlocking logic is wired into the relays so that a route can be set only if all conditions—such as route availability, point positions, and track vacancy—are safe. Once a route is accepted, the relevant signals and points are latched in their correct state to prevent conflicting changes until the route is cleared and the interlocking releases them.

Safety and reliability are achieved through the failsafe nature of relay logic: loss of power or a fault generally forces the system into a safe condition, typically red signals and secured points. Track occupancy detectors provide current information to the interlocking, ensuring that a train occupying a section cannot be mistaken for a free route.

With advances in electronics, many Relaisstellwerken were gradually replaced by electronic or computer-based interlockings (ESTW) and centralized traffic control. Nevertheless, they remain a notable stage in railway signaling history, with several preserved in operation or restored as museums and demonstration installations.