Used to describe the 'gauge' (distance between the inside of the rails) on railways.
This tag can be used to specify the track width (gauge) of railways.
It is specified in millimetres (mm, 1/1,000 of a metre) as is the railway and tagging practice. Other units can be used by adding the unit after the value (e.g., gauge=2yd), but should be avoided if possible.
As the Standard gauge of 1435 mm is used on most of the world's railways, it is particulary important to add the track width for narrow gauge and broad gauge railways, and it is suggested that mappers concentrate on adding this tag to these railways. However, this does not mean adding gauge=1435 to standard gauge railways is disallowed.
Dual gauge tracks
Tracks that support two or more gauges (by using three or more rails) should have all gauges specified using semicolon (";") as a separator. Example: gauge=1000;1435.