'G Scale' versus 'G Gauge'
A railroad gauge is the distance between the railheads. Standard US trains, for example, run on rails spaced 4' 8-1/2" apart ("standard-gauge") whereas some narrow-gauge trains (serving mines, etc.) run on rails only 36" apart. Although built with standard-sized doors, a narrow-gauge train is in some other respects smaller than its standard-gauge cousin: its cars are generally narrower and shorter, allowing them to navigate often more sharply cornered tracks.
Model trains are built to represent either a real train of standard or narrow gauge. In a HO model, for example, HO track is used to represent real standard gauge and some narrower-gauge track such as N is used to represent real narrow gauge.
G model-railways depart from this and always use the same gauge. Trains are instead built in different sizes depending on whether they are intended to represent standard-gauge or narrow-gauge trains. Because of this it might be more correct to speak of "G Gauge", the consistent aspect being the gauge, 45 mm (1.772 in), and not the scale.
G scale is thus the use of 45 mm (1.772 in) gauge track to represent both real standard gauge trains and real narrow-gauge trains, originally those of the European 1000mm gauge, at 1:22.5. Other narrow-gauge trains are modelled at other scales.
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