tangent

In railway engineering, a tangent is a straight section of track between two curves. It describes the geometry of the rail alignment itself, not the motion of a train. A tangent section has zero curvature: the track runs in a constant compass direction until a transition or curve begins. On a track plan or profile drawing, tangent sections appear as straight lines, while curves appear as arcs.

Tangents serve practical purposes beyond geometry. They allow trains to run at higher speeds with less lateral force on wheels and rails. They provide natural locations for grade crossings, switches, and signal installations. A well-designed track typically alternates between tangent sections and curves; very short tangents can cause rough ride quality and accelerated wear, while excessively long tangents delay the onset of curves and complicate sight lines.

Transitions and complexity

A true tangent meets a circular curve via a transition curve, not directly. The transition (typically a clothoid or spiral) gradually increases curvature from zero at the tangent to the full curvature of the circular arc. This eases lateral acceleration on passengers and freight. On older track, tangents connected directly to circular curves, which caused a sudden jerk and promoted rail wear. Modern standards require transition curves, making the boundary between tangent and curve less visible to the eye.

Measurement of tangent sections matters. A tangent shorter than about 100 meters (330 feet) may cause problems with track layout and drainage. Conversely, tangent sections of several kilometers on level ground can cause monotony in train operation and maintenance access. Curvature is expressed in degrees per 100 meters or radius in meters; a tangent by definition has infinite radius or zero degrees of curvature.

The term comes directly from geometry: a tangent line touches a curve at exactly one point without crossing it. In railway vocabulary, the word retains this geometric meaning but refers to the entire straight section, not just a point of contact. Engineers distinguish tangent from grade (the slope of the track in the vertical plane) and from superelevation (the banking of the track at curves).