Railway Track or Permanent Way is a combination of rails, fitted on sleepers and resting on sleepers and subgrade. The track gauge or rail gauge of a rail track is defined as the clear minimum perpendicular distance between inner faces of the two rails (distance between the inner sides of the heads of the two load bearing rails that make up a single railway line). Requirements of an ideal permanent way are that both rails should be at the same level; curves should be properly designed and proper super elevation should be provided.
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.
An exception of a railway with no gauge is monorail where there is only one supporting rail.
Gauge tolerances specify how much the actual gauge may vary from the nominal gauge.
Types of gauge:
•	Broad gauge: width:1524mm to 1676 mm or 5’-6” to  5’-0’’
•	Standard gauge: width: 1435 mm ,1451 mm or 4’-8.5’’ , 4’-9.125’’
•	Meter gauge: width: 1000 mm , 1067 mm, 915 mm or 3’-6’’, 3’-3.375’’ and 3’-0’’
•	Narrow gauge: width: 762 mm , 610 mm or 2’-6” and 2’-0”
Functions of Rail: 
•	To provide continuous and level surface for movement of train.
•	To provide a smooth pathway so that friction between rail and wheel become less.
•	Serve as a lateral guide for the running of wheels.
•	Transferring the load into the sleeper.