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 vimore recent times, the term loading gauge has fallen out of use among railway professionals, since it is a purely static concept and ignor 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,
the structure gauge is larger than the loading gauge.

Loading gauges of the world

Different lines have different loading gauges. Tube lines on London Underground have the smallest loading gauge of any railway with standrd track gauge.The loading gauge differs around the world. The smallest standard gauge loading gauge is that of the London Underground's tube lines. 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 no anticipation of the future requirements for larger trains while facing huge technical challenges building railways in this period. Elsewhere in Europe, lin
British loading gauge is 9' wide by 11' high on the sides, rising to a 13'6? centre. Below platform level (the lower 3'6?) the vehicle can be no wider than 8'8?. Some lines, particularly the Hastings line, had even narrower loading gauges. By contrast the European loading gauge is usually 10'2? wide by 10'8? rising to 13'8? in the centre. The American loading gauge was 15' high and 10'6? wide. In the 1940s and 1950s, it was stretched to 16'6? high in the West to accommodate dome cars. Technically, this i'2? double-stacks) means that most lines are designed for a huge loading gauge.
Not all railways were built to standard loading gauges. Many narrow gauge railways also have a very small loading gauge in order to keep construction costs low. The choice of loading gauge represented a significant engineering decision to trade construction and maintenance costs against train size (and thus capacity), and also led to some unusual solutions to problems, including the Fairlie locomotives.

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