Unigauge

Project Unigauge, started in 1991, is an ongoing exercise by Indian Railways to convert almost all of the rail gauges in India to 1,676 mm (5 ft 6 in) broad gauge.

Narrow-gauge railways were built in India in early years as they cost less to build as they are usually lighter in construction, used smaller cars and locomotives (smaller loading gauge), as well as smaller bridges, smaller tunnels (smaller structure gauge) and tighter curves. Narrow gauge was thus used in mountainous terrain, where the savings in civil engineering work could be substantial. It was also used in sparsely populated areas where the potential demand is too low for broader gauge railways to be economically viable.

But by early 90's, the Indian Railways concluded that the advantages of uniformity and interchangeability were judged to outweigh any other possible benefits arising from the use of diverse gauges. Also narrow-gauge railways were deemed too costly to operate and maintain. Also to handle heavier, faster traffic, narrow-gauge railways were considered impractical to improve. In 1991, these led the Indian Railways to implement Project Unigauge which standardises most of the rail gauges in India to 1,676 mm (5 ft 6 in) broad gauge.

Four narrow-gauge lines which are declared as heritage lines will not be converted to broad gauge. These are:

Urban rail transit lines (except suburban rail which are already in broad gauge) which serve the urban areas were built or are being built in standard gauge only and will not be converted to broad gauge. These encompass metro, monorail and trams.

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