The necessity of constructing a tunnel may arise because of one of the following considerations.
(a) A tunnel may be required to eliminate the need for a long and circuitous route for reaching the other side of a hill, as it would considerably reduce the length of the railway line and may also prove to be economical.
(b) It may be economical to provide a tunnel instead of a cutting, particularly in a rocky terrain. Depending upon various factors, a rough calculation would indicate that for a small stretch of land the cost of constructing a tunnel is equal to the cost of a cutting in a rocky terrain.
(c) In hills with soft rocks, a tunnel is cheaper than a cutting.
(d) In metropolitan towns and other large cities, tunnels are constructed to accomodate underground railway systems in order to provide a rapid and unobstructed means of transport.
(e) A tunnel constructed under a river bed may sometimes prove to be more economical and convenient than a bridge.
(f) In the case of aerial warfare transportation through tunnels provides better safety and security to rail users compared to a bridge or deep cutting.
(g) The maintenance cost of a tunnel is considerably lower than that of a bridge or deep cutting.
However, the construction of tunnels is also disadvantageous in certain ways, as enumerated here.
(a) The construction of a tunnel is costly as it requires special construction machinery and equipment.
(b) The construction of a tunnel involves the use of sophisticated technology and requires experienced and skilled staff.
(c) It is a time-consuming process.