Last reviewed 30 Sept 2026 · 6 min read
What is gauge
Gauge is the clear distance between the inner faces of the two rails, measured a small distance below the top of the rails (of the order of 13–14 mm). It fixes the width of the vehicles, the size of the tunnels and bridges, and the capacity of the line.
| Gauge | Distance | Use |
|---|---|---|
| Broad gauge (BG) | 1676 mm | India's main gauge (over 95 % of the network) |
| Metre gauge (MG) | 1000 mm | Older lines; largely converted to BG |
| Narrow gauge (NG) | 762 mm and 610 mm | Hill railways (Darjeeling, Nilgiri, Kalka–Shimla) and some light lines |
| Standard gauge | 1435 mm | Most of the world; used for many metros and high-speed lines; some Indian metros |
Choice of gauge
Gauge is decided by traffic, cost and compatibility:
- A wider gauge gives greater stability at speed, larger vehicles and higher capacity, but costs more for formation, bridges, tunnels and land.
- A narrow gauge is cheap to build and negotiates sharp curves in hills, but has low capacity and speed.
- Uniformity of gauge across a network avoids transhipment and the delay of break of gauge; hence the national programme of gauge conversion ("Project Unigauge").
- Compatibility with the surroundings — the neighbouring country's gauge, the port and industrial sidings.
Wheel–rail interaction
- Coned wheels: the tread of a wheel is tapered (about 1 in 20). On a curve, the outer wheel rides on the larger diameter, the inner on the smaller, so the outer wheel travels farther — the axle steers itself round the curve without flange rubbing. On a straight, the wheelset centres itself between the rails.
- Flange: the projecting lip on the inside of the wheel prevents the wheel from leaving the rails; in a normal run it does not touch the rail.
- Rails inclined inward (1 in 20): the rail is tilted so that the rail head fits the wheel cone, giving a better contact and reducing wear; the sleeper has a corresponding inclined seat.
- Hunting: a side-to-side oscillation of the wheelset caused by conicity, which increases with speed; controlled by wheel profile, suspension and track quality.
- Slack (gauge widening): on sharp curves the gauge is slightly widened so that a rigid-wheelbase vehicle does not bind; the rails are also adjusted in the transitions.
Geometric parameters of the track
- Gauge — distance between the inner faces of the rails; deviation is measured from the nominal value (tightness and slackness).
- Alignment (line) — the deviation of the rails from the ideal horizontal line; measured as a versine over a chord (e.g., 10 m chord).
- Cross level (super-elevation) — the difference in height between the two rails at a section; zero on a straight and equal to the cant on a curve.
- Twist — the change in cross-level over a stated base length (for example 3.6 m); a large twist can make one wheel unload and cause derailment.
- Unevenness (longitudinal level) — the deviation of the running surface from a straight line in the vertical direction; leads to vertical accelerations and rough ride.
- Rail inclination — the tilt of the rails (1 in 20).
Permissible tolerances
Indian Railways define maintenance limits for each parameter, depending on the class of track and the speed; for example, gauge deviations of only a few millimetres, and twist and unevenness limits that become stricter with speed. The limits are used in the Track Recording Car (TRC) and oscillograph analysis to plan maintenance and to decide on speed restrictions. (Use the current Indian Railways Permanent Way Manual for the precise values.)