When selecting a gauge, there is always a trade-off between different pros and cons:
- Narrow Gauge:
Pros: tighter turning radius, lower cost, less demanding right-of-way and construction
Cons: lower speed, less stability, less load carrying capacity
- Broad Gauge:
Pros: Higher speed, stability and capacity
Cons: wider turning radius, higher cost, more demanding right-of-way and construction
One generally wants higher speed, greater stability and larger capacity, and one wants lower cost and tighter turning radius, but there is an inverse relationship between these factors.
In addition, there are other constraints, such as the load-carrying capacity of axles, which can be problematic with an excessively wide gauge.
Narrow gauge railways usually cost less to build because they are usually lighter in construction, using smaller cars and locomotives (smaller loading gauge), as well as smaller bridges, smaller tunnels (smaller structure gauge) and tighter curves. Narrow gauge is thus often used in mountainous terrain, where the savings in civil engineering work can be substantial.
It is also used in sparsely populated areas, with low potential demand, and for temporary railways that will be removed after short-term use, such as for construction, the logging industry, the mining industry, or large-scale construction projects, especially in confined spaces (see Temporary way – permanent way).
Broader gauge railways are generally more expensive to build and require wider curves, but are able to handle heavier and faster traffic.
There is no single perfect gauge, because different environments and economic considerations come into play.
A narrow gauge is superior if one’s main considerations are economy and tight curvature (e.g. Japan relies on the 3’6″ gauge due to its mountainous terrain).
For direct, unimpeded routes with high traffic, a broad gauge may be preferable (which is why the BART train system uses Indian/5’6″ gauge). The Standard, Russian, and 4’6″ gauges are designed to strike a reasonable balance between these factors.
In addition to the general trade-off, another important factor is standardization.
Once a standard has been chosen, and equipment, infrastructure, and training calibrated to that standard, conversion becomes difficult and expensive.
This also makes it easier to adopt an existing standard than to invent a new one.
This is true of many technologies, including railroad gauges. The reduced cost, greater efficiency, and greater economic opportunity offered by the use of a common standard explains why a small number of gauges predominate worldwide.