threerail

Threerail, or three-rail, refers to railway track or power systems that use three conductive rails running parallel along the track. In these arrangements, the outer rails carry one electrical polarity (or circuit return) while a center rail provides the opposite polarity (or a separate supply). The center rail is typically insulated from the outer rails and connected to a dedicated power source or control circuitry.

In model railroading, three-rail systems are most closely associated with Lionel and other American toy train brands that became popular in the mid-20th century. The standard setup uses two outer rails and a center conductor, allowing locomotives and rolling stock to draw power from multiple rails. This configuration simplified wiring and polarity handling for hobbyist layouts and supported a range of voltage schemes and control methods used by traditional power packs and accessories.

Historically, three-rail track also appeared on some real electric traction networks, including certain interurban and streetcar systems, where a center conductor provided power in addition to the two return paths. These implementations varied in detail and were more common in earlier eras of electric railway development; they are comparatively rare in modern mainline operations.

Today, threerail systems persist primarily in hobbyist contexts, where vintage and contemporary three-rail equipment continues to be manufactured and operated on dedicated layouts. In the broader rail industry, two-rail systems are far more prevalent, with three-rail arrangements largely maintained for historical, nostalgic, or specialized modeling purposes.