Rail gauge

Rail gauge is the distance between the inner sides of the heads of the two load bearing parallel rails that make up a single railway line. Sixty percent of the world's railways use a standard gauge of 1,435 mm (4 ft 8 1⁄2 in). Wider gauges are called broad gauge; smaller gauges, narrow gauge. Break-of-gauge refers to the meeting of different gauges. Some stretches of track are dual gauge, with three or four rails, allowing trains of different gauges to share them. Gauge conversion can resolve break-of-gauge problems. An exception of a railway with no gauge is monorail where there is only one supporting rail. Some electrified railways use non load bearing third rail and occasionally a '4th' rail. These additional rails are positioned between or outside the 'running rails' to feed and return electrical current, they do not define the rail gauge.