Dual gauge

A dual-gauge railway is a line of track that provides for trains of two separate gauges. A mixed-gauge railway provides for more than two separate gauges, but is also a term sometimes used to denote dual-gauge. A dual-gauge track configuration usually consists of three rails, two 'vital' (or gauge-) rails, one for each gauge, plus a 'common rail'--although at times, commonality is not possible and four rails are required (the two separate tracks of each gauge are placed within each other). In an ordinary three-rail dual-gauge configuration, the two adjacent outer rails give provide each of the gauges, while the single outer rail is common to trains of both gauges operating over it. This configuration is not to be confused with the electric current traction rail (third rail) or a check or guard rail.