What defines the track gauge on a railway?

The track gauge on a railway is defined as the distance between the inner sides of the rails. This measurement is crucial because it determines the stability and compatibility of the trains that operate on those tracks. A standard gauge, which is 56.5 inches (1,435 mm), is widely used, but there are also various other gauges employed around the world for different purposes. The gauge influences the design of rolling stock, the physics of train movements, and the overall safety of railway operations.

In contrast, the other options relate to different aspects of railway operations. The length of train cars pertains to the capacity and design of the trains but does not influence track gauge. Height of platforms is relevant to passenger boarding and alighting but is independent of track gauge. The weight of trains is an important consideration for track construction and maintenance but does not define the track gauge itself. Therefore, the correct answer emphasizes the specific physical measurement that directly defines the gauge of the railway tracks.