In rail transport, track gauge (in American English, alternatively track gage) is the distance between the two rails of a railway track. All vehicles on a rail network must have wheelsets that are compatible with the track gauge.

The distance or "gap" between the two inner edges of the iron rails on a single track is technically called the Track Gauge. Globally, the most common is the "Standard Gauge," which measures 1,435 mm (4 ft 8.5 in). However, terminology differs slightly when referring to the spaces around the rails. In British railway parlance, the space between the two rails of a single track is colloquially known as the "Four-foot," while the safety gap or space between two sets of parallel tracks is called the "Six-foot." There is also the "Cess," which is the area outside the tracks used for drainage and worker safety. If you are referring to the physical space between the bottom of the rail and the ground, that is the "ballast bed." Understanding these terms is vital for railway engineering and safety; for instance, "gauge" determines which trains can run on which networks, as seen in the "Broad Gauge" systems of India or the "Narrow Gauge" lines used in mountainous regions.