Switch
The trackwork assembly with movable point blades that diverts a train from one route to another — known as a turnout or set of points.
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A switch — or turnout, or, in British and Commonwealth usage, a set of points — is the trackwork assembly that allows a train to be diverted from one route onto another. It consists of two tapered movable rails called switch rails or point blades that pivot at one end and slide horizontally at the other, snapping against either of two stock rails depending on which route is selected. The frog and check rails downstream complete the assembly, but the switch proper is the moving piece — the part that controls which way the wheels go.
Switches are described by the angle of divergence, written as a ratio (1-in-8, 1-in-10, 1-in-20). A 1-in-8 turnout diverges sharply and is rated for perhaps 15-25 mph on the side track; a 1-in-20 spreads the divergence over a long lead and supports 40 mph or more. High-speed crossovers between mainlines on 300 km/h networks use turnouts up to 1-in-65 with switch leads tens of metres long.
Movement is controlled by a switch machine — historically a manual ground throw with a lever and a counterweight, today an electric or hydraulic actuator that drives both blades together via a stretcher bar. In an interlocked plant the machine is tied into the signalling system so that the route cannot be cleared until the blades are proven fully home and locked; conversely, the blades cannot be moved while a train is detected over them. Hand-thrown switches still exist on industrial spurs and some yard tracks, but every mainline switch on a modern operating railway is interlocked.
For railfans, the most distinctive switch sound is the metallic clunk of the actuator at a remote interlocking — often audible long before any train, and a reliable cue that a movement is being lined up nearby.