A loading gauge is the envelope or contoured shape within which all railway vehicles, engines, coaches, and trucks must fit. It is dictated by the size of tunnels, height of bridges and shape and height of platforms. It varies between different countries and may also vary on different lines within a country. For example, metro trains might have smaller loading gauge than conventional railway to allow smaller tunnels. In that case metro trains may run on conventional tracks, but not vice versa.

In more recent times, the term loading gauge has fallen out of use among railway professionals, since it is a purely static concept and ignores other factors affecting clearance. Instead, the terms dynamic envelope or kinematic envelope are used. Factors such as suspension travel, overhang on curves (at both ends and middle), lateral motion on the track, etc. are just as important as the vehicle's static profile. All these factors must be considered in determining whether the moving rail vehicle will fit within allowed clearances.

Strictly speaking:

• loading gauge is maximum size of rolling stock.
• structure gauge is minimum size of bridges and tunnels.
• There has to be some separation between the above two.

Loading gauges of the world

The loading gauge differs around the world. The smallest standard gauge loading gauge is that of the London Underground's deep lines ; early Victorian engineers only had the technical and financial capabilty to build small bore deep tunnels. The largest loading gauge is that of the Channel Tunnel between Great Britain and France.

The loading gauge on the main lines of Great Britain, where rail transport started, is quite small as early engineers had to save money on construction. Elsewhere in Europe, lines tend to conform to the slightly larger Berne gauge and loading gauges in the United States tend to be larger still. The Russian loading gauge is also large.