headchair

A headchair is a cast iron or fabricated steel component that forms the fixed pivot point and support structure at one end of a railway switch (also called a turnout or point). The moveable rails, known as switch rails or points, rest their heel ends on the headchair and are hinged to it, while the fixed stock rails are bolted directly to the headchair casting. This arrangement allows the switch rails to swing laterally between two positions, directing a train onto either the main line or the branch line.

The headchair must accommodate significant lateral and vertical loading as trains pass through the switch at speed. It sits at the narrow end of the diamond-shaped plan view of the turnout, and its design determines the guard rail geometry and the angle of the switch taper. Most headchairs are monolithic castings weighing between 200 and 400 kilograms, though modular headchairs bolted from multiple steel plates are used in some modern high-speed installations.

Wear accumulates at the pivot point where the switch rails hinge, and sediment, leaves, and ice can lodge in the gap between the heel of the switch rail and the headchair seat. This accumulation prevents full closure and creates a gap that can catch wheel flanges, causing derailment. Rail crews regularly clean and lubricate this interface, and the gap tolerance is typically held to within 5 to 10 millimetres when the switch is properly positioned.

The term headchair derives from the anatomical parallel: it is the 'head' or fixed end of the switch mechanism, and it 'chairs' or supports the moving rails as they pivot. The corresponding component at the other end of the switch, where the point rails converge, is called the toe or point. The headchair is a purely passive component; the actual movement of the switch rails is driven by a separate mechanical or hydraulic actuator located off to the side.

Defective headchairs cannot be repaired in service and must be replaced as a complete unit, involving temporary line closure and rail removal. Casting voids, surface cracks, or severe wear of the pivot bearing seat are cause for replacement. On older lines, original cast iron headchairs from the 19th century remain in service, though increasingly they are being swapped for modern ductile iron or forged steel versions that resist impact and fatigue damage more effectively.