turnout

A turnout is the mechanical assembly that allows a train to be routed from one track onto another. It consists of two moveable rails, called points or switches, that pivot to guide wheels toward either the main line or a diverging branch. The points are held in position by a switch machine driven hydraulically, pneumatically, or electrically. Without turnouts, every train would be locked to a single path; with them, a rail network becomes a routing system.

The standard turnout comprises the points themselves, a frog (where the running rails of the two routes intersect and cross), guard rails to keep wheels aligned during the transition, and the switch mechanism. A simple turnout allows trains to branch left or right from a main line. A diamond or crossing allows two lines to pass over each other. More complex assemblies like slip switches or double crossovers combine multiple elements to save space and permit movement in several directions simultaneously.

Maintenance and Failure Modes

Turnouts are wear items. The points themselves erode from repeated wheel contact; the frog develops gaps that can cause derailment or rough riding; the switch machine's linkages can become sluggish or fail entirely. Ice, snow, and debris in the point mechanism can prevent proper closure, leaving the turnout partially open and unsafe. Tamping and realignment of the track bed under a turnout is more frequent than elsewhere on the line because the constant lateral forces at the frog and the asymmetric loading on the points cause accelerated settlement.

The term turnout, borrowed from coaching terminology where it referred to a carriage and team, became standard in British rail usage. Americans often call the same thing a switch, while the points themselves are sometimes called the switch rails. The frog is named for its four-legged shape resembling the amphibian. Numbered turnouts like "Turnout 47" help dispatchers and maintenance crews reference exact locations on a line without ambiguity.

The speed at which a train can pass through a turnout depends on the frog angle, which is expressed as a ratio such as 1 in 8 or 1 in 12. A tighter angle (1 in 8) is slower but takes less track length; a gentler angle (1 in 12) permits higher speed but demands more real estate. Passenger lines typically use gentler angles to reduce discomfort and wear; freight and industrial sidings accept tighter angles to fit into congested yards.