Axle track
In automobiles (and other wheeled vehicles which have two wheels on an axle), the axle track is the distance between the hub flanges on an axle. Wheel track, track width or simply track refers to the distance between the centerline of two wheels on the same axle. In the case of an axle with dual wheels, the centerline of the dual wheel assembly is used for the wheel track specification. Axle and wheel track are commonly measured in millimetres or inches. Common usage. Despite their distinct definitions, "axle track", "wheel track" and "track width" are frequently used interchangeably , normally to refer to the distance between the centerline of the wheels. For a vehicle with two axles, the measurements can be expressed as "front track" and "rear track". For a vehicle with more than two axles, the axles are normally numbered for reference. Offset wheels. In vehicles with offset wheels, wheel track is distinct from axle track because the centreline of the wheel is not flush with the hub flange. If wheels of a different offset are fitted, the wheel track changes but the axle track does not. Rail. In the case of a rail wheelset the axle track  is called wheel gauge and is measured from wheel flange reference line to wheel flange reference line on the wheels of a rail car or tram axle. The wheel gauge of a rail vehicle must be compatible with the track gauge of the network it runs on. Model rail. Model railway elements such as track, rolling stock and locomotives are categorised by their wheel or track gauge. An OO gauge model locomotive, for example, has a wheel gauge of 16.5mm.