Gauges

Track gauge is the distance between the inner faces of the two rails, measured a short distance below the top of the rail. It is the most basic dimension of a railway: rolling stock built for one gauge cannot run on another without changing wheelsets or bogies, so gauge decides which trains can run where.

India historically built three main gauges. Broad gauge, 1 676 mm, was chosen for trunk routes; metre gauge, 1 000 mm, was used for cheaper secondary lines; and narrow gauges of 762 mm and 610 mm served hill and feeder lines. Breaks of gauge forced passengers and freight to transfer, so a long programme of gauge conversion has brought most of the network to broad gauge, leaving some narrow-gauge hill lines as heritage routes.

Standard gauge, 1 435 mm, is used by most railways worldwide and by most modern metro and high-speed lines, including high-speed rail in India, because rolling stock and systems for it are widely available. A wider gauge allows larger, more stable vehicles and higher axle loads; a narrower gauge allows sharper curves and cheaper construction in hilly terrain.

Gauge is held within tight tolerances by fastenings and sleepers, and widening or tightening on curves is controlled. Track geometry recording cars measure gauge along with alignment, cross level and twist. In calculations such as cant, Indian Railways uses the distance between the centres of the rail heads, taken as 1 750 mm for broad gauge.