Terminology
The terms standard gauge, broad gauge and narrow gauge do not have any fixed meaning. A "standard" gauge is only standard in a geographical region where it is dominant, although without qualification it is generally understood to refer to a nominal gauge of 1435mm or 4 ft 8½in (or a locally determined gauge close to those values). An infrastructure owner would be ill-advised to order track materials simply as "standard gauge", but would normally specify the required critical dimensions of the components.
Broad gauge and narrow gauge refer to track gauges greater or less than the generally adopted standard: they are relative terms.
In the British area of influence in southern Africa, the gauge of 3 ft 6in was widely adopted, and became known as the Cape gauge.
The terms structure gauge and loading gauge have nothing to do with track gauge. They are both widely used, but imprecise, terms. Structure gauge describes the cross-section envelope into which new or altered structures (bridges, lineside equipment etc) must not encroach. Loading gauge is the corresponding cross-sectional profile within which rail vehicles and their loads must be contained. If an exceptional load, or a new type of vehicle, is being assessed for freedom to run on a certain route, it must conform to the route's loading gauge.
Historically a space between the two profiles was required to allow for dynamic effects, extreme wear and surveying tolerances, but in current practice all tolerances are incorporated into the vehicle operating profile and no other allowance is necessary.
Nowadays there are other parameters that must be assessed for decisions on interoperability; these include electro-magnetic compatibility, compliance with control system parameters, and of course axle load and loading envelope in general.
In British practice, the general space between the rails of one track is colloquially referred to as "the four-foot way"; this is contrasted with the space between two tracks: "the six-foot way". These descriptions are simply convenient phrases approximately relating to the respective dimensions.
Read more about this topic: Rail Gauge