Which of the following factors govern the choice of the gauge? (i) Volume and nature of traffic (ii) Speed of train (iii) Physical features of the countryThe correct answer is

Railway engineering is a multi-faceted engineering discipline dealing with the design, construction and operation of all types of rail transport systems. (Source: Wikipedia)

Engineering subjects that are part of railway engineering include:

Mechanical engineering.
Command, control & railway signalling. Office systems design. Data center design. SCADA. Etc
Electrical engineering. Energy electrification. Third rail. Etc
Civil engineering. Permanent way engineering.
Light rail systems.
On-track plant.
Rail systems integration.
Train control systems. Cab signalling.