Gauge

Gauge, more precisely track gauge, is the distance between the gauge faces of the two running rails, measured at a specified level below the top of the rail. In European railway practice, track gauge is commonly measured 14 mm below the top of the rail, although the precise measurement position should always follow the standard applicable to the railway system.

Track gauge is one of the fundamental geometric parameters of a railway. It establishes the relationship between the track and the wheelsets of the rolling stock and is therefore fundamental to vehicle compatibility, wheel–rail interaction and safe railway operation.

The most widely used railway gauge worldwide is Standard Gauge (1,435 mm). However, railway networks use several different gauges due to historical development, geography, engineering requirements and network standards.

Railway gauges are generally described as:

Standard Gauge — 1,435 mm: The internationally predominant gauge, widely used for conventional railways, high-speed rail and metro systems.

Broad Gauge — greater than 1,435 mm: Examples include 1,520/1,524 mm gauges used across parts of Eastern Europe and Eurasia, and 1,676 mm Indian Broad Gauge.

Narrow Gauge — less than 1,435 mm: A category rather than a single dimension. Common examples include 1,067 mm, 1,000 mm, 914 mm, 800 mm, 762 mm and 600 mm.

Metre Gauge — 1,000 mm: A specific and widely used form of narrow gauge found on railway networks in several regions of the world.

Track gauge directly influences the design and performance of wheelsets, bogies, rails, sleepers, fastening systems, switches and crossings, track geometry and maintenance tolerances. Rolling stock designed for one track gauge generally cannot operate directly on a railway of another gauge unless specific technical arrangements are provided.

Maintaining the specified track gauge is essential for safe wheel–rail interaction. Gauge is therefore controlled within defined construction and maintenance tolerances. Excessive gauge widening or narrowing can adversely affect wheel–rail contact, vehicle dynamics, ride quality and, in severe circumstances, derailment safety.

Track gauge can also vary locally within permitted limits due to factors such as track design, curvature, wear, construction tolerances and maintenance condition. The applicable infrastructure standard defines the nominal gauge and the permissible deviations from it.

Where railway networks with different gauges meet, a break of gauge creates an interoperability interface. Solutions may include:

Passenger or freight transfer
Bogie exchange
Variable-gauge wheelsets
Dual- or mixed-gauge track
Transshipment of freight

Some railway corridors use dual-gauge or mixed-gauge track, where additional rails allow rolling stock designed for different track gauges to operate over part of the same infrastructure.

It is important not to confuse track gauge with loading gauge or structure gauge. Track gauge defines the relationship between the two running rails; loading gauge defines the permissible vehicle and load envelope, while structure gauge establishes the clearance envelope that surrounding infrastructure must maintain.