chair

A chair is a cast-iron casting that cradles and secures a bullhead rail to a wooden or concrete sleeper. The rail sits in a saddle formed into the top of the chair, and a wedge or key driven from the side clamps the rail firmly in place. This method of rail attachment was standard on British railways and many other systems using bullhead rail profile, where the rail head sits proud of the rail foot, requiring a separate component to hold it down.

The chair itself bolts or is spiked to the sleeper, usually with two or four fixings depending on design. Early chairs used wooden wedges; later designs employed metal keys or elaborate toggle mechanisms. The casting must withstand repeated impact from wheels and lateral forces from curves and braking. Cast iron was chosen for its ability to absorb vibration, its brittleness being acceptable in a component that rarely bends but must not deform under compression.

Standard chairs on a main line carry significant rail loading and require replacement when the saddle wears into a groove or the key mechanism becomes slack. Chairs designed for branch lines or sidings have lighter construction. Some chairs incorporated a check rail seat for curved track, adding a second smaller saddle to guide the flange. Over decades, chairs accumulate wear, and the wedge may work loose, allowing the rail to shift and develop a hollow spot that causes noise and accelerates further damage.

The chair represents a transitional technology between the old practice of spiking rail directly to timber and modern continuous welded rail fixed with resilient clips. As bullhead rail has largely been superseded by flat-bottomed rail on newly laid track, chairs have become less common on trunk routes, though thousands remain in service on heritage railways and historic main lines. The term persists in railway vocabulary even where the component no longer appears.