The total gap on both sides between the inside edges of wheel flanges and gauge faces of the rail is kept as

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.