loop

A loop is a continuous curved section of track that allows a train or tram to change direction 180 degrees and return the way it came, without requiring the operator to reverse the vehicle or uncouple and reposition it. The train enters the loop, travels through a continuous arc, and exits facing the opposite direction on the same or parallel track. This is standard infrastructure at terminal stations where no reversing sidings exist or where rapid turnaround is needed.

The geometry of a loop must accommodate the vehicle's turning radius, which depends on wheelbase and track gauge. Narrow-gauge systems like heritage railways and many urban tram networks use tighter loops, sometimes with radii under 30 metres. Standard-gauge heavy rail loops are considerably larger, with radii typically 200 metres or more. The track itself remains single or double gauge throughout; the loop does not involve any gauge change or switch mechanism beyond the gentle curve itself.

Variants and constraints

A balloon loop is the most common form: a circular or near-circular track attached to one end of a line. A turning loop or reversing loop may describe variants where the geometry allows turning but is not purely circular. In model railways, reversing loops require special electrical wiring to avoid short circuits, but full-scale loops present no such problem. On steep gradients, loop construction becomes difficult and expensive, which is why mountain railways and steep branch lines often rely on reversing or zigzag (switchback) methods instead.

The loop must be clear of points, signals, and platform infrastructure. Track maintenance and inspection access must be planned carefully. Snow and leaf accumulation can occur in the tight sections of a loop, requiring regular cleaning. Signalling within a loop is straightforward since the train follows a fixed path; drivers cannot take diverging routes as they would at a junction.

Loops are economical where space permits and turnaround frequency is high. They are now standard at most modern tram termini in Europe and Asia, and were historically built at railway branch termini before modern depot layouts and reversing sidings became standard. On heritage and light railway systems, a loop often serves as both a scenic feature and a practical necessity.