Editing Track gauge (section)

==Nominal track gauge==
The nominal track gauge is the distance between the inner faces of the rails. In current practice, it is specified at a certain distance below the rail head as the inner faces of the rail head (the ''gauge faces'') are not necessarily vertical. Some amount of tolerance is necessarily allowed from the nominal gauge to allow for wear, etc.; this tolerance is typically greater for track limited to slower speeds, and tighter for track where higher speeds are expected (as an example, in the US the gauge is allowed to vary between {{cvt|4|ft|8|in|mm}} to {{cvt|4|ft|10|in|mm}} for track limited to {{cvt|10|mph|km/h}}, while {{cvt|70|mph|km/h}} track is allowed only {{cvt|4|ft|8|in|mm}} to {{cvt|4|ft|9+1/2|in|mm}}. Given the allowed tolerance, it is a common practice to widen the gauge slightly in curves, particularly those of shorter radius (which are inherently slower speed curves).

Rolling stock on the network must have running gear ([[Wheelset (rail transport)|wheelsets]]) that are compatible with the gauge, and therefore the gauge is a key parameter in determining interoperability, but there are many others – see below. In some cases in the earliest days of railways, the railway company saw itself as an infrastructure provider only, and independent hauliers provided wagons suited to the gauge. Colloquially the wagons might be referred to as "four-foot gauge wagons", say, if the track had a gauge of four feet. This nominal value does not equate to the flange spacing, as some freedom is allowed for.

An infrastructure manager might specify new or replacement track components at a slight variation from the nominal gauge for pragmatic reasons.