chord

A chord is a connecting track that joins two main railway lines that cross one another at different elevations, allowing trains to move between them without using the grade separation structure itself. In a typical setup, two lines might cross at a junction where one line runs above the other on a bridge or viaduct; the chord is a fourth track, or sometimes a pair of tracks, that spirals or angles upward from the lower line to meet the upper line, creating an alternative routing path.

Chords are most commonly found at major rail hubs and interchange points where traffic between two separate networks must be accommodated. A train that needs to move from the lower line to the upper line can use the chord rather than routing through the main junction throat. This reduces congestion at the primary crossing and allows more flexible traffic management. The geometry of a chord is typically more gradual than a direct connection would be, since rail grades are limited; typical maximum grades for freight lines are around 1.5 to 2 percent, and a chord must respect these constraints while gaining elevation over a sufficient distance.

Chord layouts and operational use

Chords come in two principal forms: single-track chords, which restrict movement to one direction and are common where traffic flow is heavily unidirectional, and double-track chords, which permit two-way movement and are used where more complex traffic patterns exist. Some major yards have multiple chords serving different pairs of lines. The chord itself is often signaled as a distinct route, with its own signals and interlocking logic to prevent conflicts with movements on the main lines.

The term 'chord' derives from geometry: just as a chord of a circle connects two points on the circumference without following the arc, a railway chord connects two lines directly without following the main junction structure. This makes the chord a practical shortcut for certain train movements, even though it adds trackwork and complexity to the yard or junction layout.

Maintenance of chords presents particular challenges because they typically see lower traffic volume than main lines but must still be kept to full service standards; track geometry degradation on a lightly-used chord can be harder to predict and schedule. Snow, drainage, and vegetation management are also more difficult on ascending chords, since they are often in less-trafficked portions of the yard where resources may be spread thin.