Rail gauge is defined as

• A. The distance between inner faces of the rails; standard gauge is 1,435 mm (4 ft 8 1/2 in); broad and narrow gauges exist in other systems.
• B. The distance between inner faces of the rails; standard gauge is 1,400 mm; other systems vary.
• C. The distance from the center of one rail to the center of the next rail.
• D. The length of a rail section.

Answer: (A)

Gauge is the distance between the inner faces of the two rails. This spacing determines which wheelsets can run on a track and how they fit through curves and transitions. The standard gauge is 1,435 millimeters (4 ft 8 1/2 in), and many regions use this as the baseline, while other systems employ broader or narrower gauges. Measuring center-to-center would mix in rail thickness and doesn’t reflect the actual gap the wheels ride on, so it isn’t used as the gauge. The length of a rail section has no bearing on the track gauge.