The Indian Railways operate in four different gauges-(i) broad gauge (width 1.676 mares), (ii) meter gauge (1.000 meter), (iii) narrow gauge (0.762 meter), and (iv) lift gauge (0.610 meter). These gauges were originally devised during the colonial days keeping in mind the volume of traffic and goods movement, importance of the places connected and the nature of terrain.
The port cities of Kolkata, Mumbai, and Chennai, the gate ways for the export of raw materials and the import of manufactured goods along with the inland centre of Lahore were inter-connected by broad gauge and constituted the basic framework of the rail system during the British regime. Some broad gauge branch routes like Allahabad-Itarasi-Bhusawal etc were also constructed to link places of industrial and strategic importance.
Areas lying beyond the main framework attracted only main and branch lines of the meter gauge. Hence the plain lying north of the Ghaghra- Ganga alignment, whole of Rajasthan, Gujarat and the southern Indian plain and the state of Hyderabad came under the meter gauge rail line. Some of the important meter gauge rail lines were Madras (now Chennai) Tuticorin along the east coast, Ahmadabad- Delhi and Delhi-Lucknow-Varanasi loop line.
Narrow gauge rail lines were laid down in the hilly, barren and thinly populated areas of less economic importance. Pathankot-Manali-Kalka-Shimla, Siliguri-Darjeeling.Mettupalayam-Ootacamund includes some important rail links under the system.
These triple gauges exhibit the hierarchical importance of the rail routes. The double track broad gauge today linking the metropolitan cities and major ports comes on the top followed by single track broad gauge branch line on peripheral trunk routes, meter gauge double track lines, meter gauge single track routes and narrow gauge rail lines.
After Independence it was realised that different gauges of the railways are creating problems in the movement of goods and traffic. Hence, it was decided to bring the whole railway network under single broad gauge. Here again priority was given to first convert meter gauge trunk routes.
Out of the total route length of 62,367 km in 1990-91 55.93 per cent was under broad gauge, 37.55 per cent under meter gauge and 6.52 per cent under narrow gauge. During 2003-04 the total rail length increased to 63,221 km of which 74.04 per cent was under broad gauge (46.807 km), 21.02 per cent (13,290 km) under meter gauge, and remaining 4.94 per cent (3,124 km) under narrow gauge. Since the process of gauge conversion is very slow owing to the paucity of funds it will take many more years to bring the total railway system under single gauge. Some important conversion routes include-Suratgarh- Bathinda, Ernakulam-Thiru vananthapuram, Allahabad City-Varanasi-Bhatni-Gorakhpur, Samastipur-Darbhanga, Guvvahati-New Bongaigaon- Katihar-Barauni-Gorakhpur-Lucknow, Guntakal- Dharmawaram, Manmad-Prabhani-Purli-Vaijnath, Guntur-Machrela, Viramgarh-Okha-Porbandar, and Delhi-Sabarmati etc.