Nominal Track Gauge
The nominal track gauge of a railway system is the distance between the inner faces of the rails of an individual track. In current practice it is specified to be measured at a certain distance below the rail head as the inner faces of the rail head (referred to as the gauge faces) are not necessarily vertical.
The rolling stock used on the network must have running gear (wheelsets) that is compatible with the track gauge of the network, and therefore the track gauge is a key parameter in determining interoperability. (In present practice there are many others—see below.) In many cases in the earliest days of railways, the railway company saw itself as an infrastructure provider only, and therefore independent hauliers needed to provide wagons suited to the track gauge. Colloquially the wagons might be referred to as "four feet gauge wagons", say, if the infrastructure had a track 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.
Read more about this topic: Rail Gauge
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