The axle track in automobiles and other wheeled vehicles which have two or more wheels on an axle, is the distance between the centreline of two roadwheels on the same axle, each on the other side of the vehicle. In a case of the axle with dual wheels, the centerline in the middle of the dual wheel is used for the axle track specification.
In a car, or any vehicle, with two axles, this will be expressed as "front track" and "rear track".
However the front wheels and/or rear wheels on either side of a vehicle do not necessarely have to be mounted on the same axle for the distance that they are apart to be called the "track".
In the case of a rail wheelset the "track" is called "wheel gauge" and is measured from wheel flange reference line to wheel flange reference line.