Rail gauge

Rail gauge is the distance from the inside of one rail on a railroad track to the inside of the other. 
Most tracks use a standard gauge of 1,435 mm (4 ft, 8 1/2 in). Wider gauges are called broad gauge (1676mm), smaller gauges are called narrow gauge(762mm or 610mm). 
Broad gauge may be used where the track needs to have big things moving on it. Narrow gauge, such as metre gauge (1000mm), is used to build railroads for less money, because the smaller track costs less. Places where different gauges meet are called break of gauge.