required to do what a single train would normally accomplish. When traffic passes mainly in one direction, full wagons taken to the border have to be returned as empties, and a train of empty wagons has to be brought to the break of gauge from the other side to fetch the cargo. Congestion is also caused by unloading and reloading. The problem is worsened when there is a disparity between the capacity of locomotives and vehicles on the two gauges: typically, one broad-gauge trainload needs three narrow-gauge trains to carry. Dual gauge (three rails) Constructing dual-gauge track with three rails is possible when the two adjacent rails can be separated by at least the space required by rail fastening hardware such as spikes and or rail clips – typically . If the two gauges are closer than that, four rails must be used. Depending on the weight of rails (heavy rails are bigger), the practicable difference between the two gauges is to . Dual gauge (four rails) There are many places where gauge dimensions of two different railway systems are too close to allow a three-rail configuration, including: and (common in Africa, a legacy respectively of French/Belgian and British railway practice) and (common in South America) and (common where broad-gauge railways of former satellite states of the Soviet Union meet European or Chinese standard gauge). In such cases, four rails are needed for dual gauge. In addition, four rails may result from an engineering preference when three rails would suffice: an example is on the Chemin de Fer de la Baie de Somme (Somme Bay railway), which combines standard and metre gauge – different, well within the parameters for three rails. Four rails can also be deployed where it is essential to have the centre-line of all rail vehicles aligned with the centre-line of the track. Examples are: in constricted tunnels passenger platforms, where for reasons of safety and comfort the step taken by the passenger must be within uniform limits on turntables, which must present identical positioning of rails at both ends, requiring all four rails to be aligned to the central axis. Triple gauge In rare situations, three different gauges may converge on to a rail yard and triple-gauge track is needed to meet the operational needs of the break-of-gauge station – most commonly where there is insufficient space to do otherwise. Construction and operation of triple-gauge track