PSB

A power signal box is a centralized control point where trained operators manage train movements across a section of railway line using electrical or electronic equipment rather than mechanical levers and interlocking frames. The PSB receives information from trackside sensors and signals, allows operators to set routes and clear signals by pressing buttons or using a control panel, and automatically enforces safety rules through electrical interlocking logic. A single PSB can control dozens of miles of track, many junctions, and multiple signaling aspects simultaneously.

Power signal boxes emerged in the early twentieth century as railways grew too large and complex for traditional mechanical boxes to handle efficiently. A mechanical box might control only a few miles of line with perhaps twenty levers operated by hand; a PSB of modest size can manage five times that distance. The electrical interlocking prevents conflicting routes being set, such as directing two trains toward the same track section, without requiring the operator to hold every rule in their head. Modern PSBs use solid-state electronics or computer-based systems, whereas earlier installations used electrical relays and linear arrangement of contacts.

Operators at a PSB work from a control desk displaying a schematic diagram of the line section on a mimic board or screen. They identify approaching trains, review their scheduled movements, and request route clearance through the system. Pressing a route button initiates a sequence of automatic lever operations: points move to the correct position, conflicting signals remain red, and only when the route is fully set and safe does the signal clear to green. All route setting is logged, providing a record of every movement for operational review and accident investigation.

Power signal boxes require reliable electrical supply, typically backed by batteries or standby generators to maintain safety during power loss. The control cables running from the PSB to lineside equipment (points motors, signal lamps, track circuits) must be protected from damage and corrosion. Larger installations may split the controlled section between two or more PSBs, with handover procedures at the boundary to coordinate train movements across both areas.

The term "power" distinguishes these installations from mechanical boxes and from older semi-mechanical designs that used manual levers but electrical locking. Some railways still operate mechanical boxes on lightly used branches, but PSBs have been the standard for major routes for decades. A PSB is not the same as a level crossing control unit or a simple automated signal system; it is a comprehensive traffic management tool requiring trained operators and regular maintenance of complex equipment.