Loading gauge

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,

the structure gauge is larger than the loading gauge.

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 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, lines tend to conform to the slightly larger Berne gauge and loading gauges in the United States tend to be larger still. The Russian the Chinese loading gauges are also large.

British loading gauge is 9′ (2743 mm) wide by 11′ (3353 mm) high on the sides, rising to a 13′-6″ (4115 mm) centre. Below platform level (the lower 3′-6″ or 914 mm) the vehicle can be no wider than 8′-8″ (2642 mm). Some lines, particularly the Hastings line, had even narrower loading gauges. By contrast the European (Berne) loading gauge is usually 10′-2″ (3150 mm) wide by 10′-5″ (3175 mm) rising to 14'-6″ (4280 mm) in the centre. This is a clearance envelope on a curve of 250 m (820'-2.5") radius.

The American loading gauge for freight cars was 15′-1" (4597 mm) high and 10′-8″ (3251 mm) wide with 41'-3" (12.573 m) truck (bogie) centers (AAR Plate B and B-1) as well as 15'-6" (4724 mm) high and 10′-8″ (3251 mm) wide with 46'-3" (14.097 m) (AAR Plate C and C-1) on a 13° curve, a circular segment formed by a 100' (30.48 m) chord, which in this case means a radius of 441.684' (134.625 m), see law of sines, examples. Technically, 15'-1" (Plate B) is still the maximum and the circulation of 15'-6" (Plate C) is somewhat restricted, but the extreme frequency of excess-height rolling stock, at first ~18′ (5486 mm) piggybacks and hicube boxcars then later autoracks, airplane parts cars as well as 20′-2″ (6147 mm) high double-stacked containers in container well cars, means that many, but not all, lines are now designed for a huge loading gauge. The width of these extra height cars is covered by Plate C-1. Height restrictions apply to the Long Island Rail Road (LIRR) which can not even handle the 15'-1" height and to Amtrak's Northeast Corridor.

The standard North American passenger car is 10'-6" (3200 mm) wide by 14'-6" (4420 mm) high and measures 85'-0" (25.908 m) over coupler faces with 59'-6" (18.136 m) bogie (truck) centers or 86'-0" (26.213 m) over coupler faces with 60'-0" (18.288 m) bogie (truck) centers. In the 1940s and 1950s, the American passenger car loading gauge was increased to a 16′-6″ (5029 mm) height in the West to accommodate dome cars and later superliners and other double-decker trains. Amtrak's Northeast Corridor, especially Pennsylvania Station (New York City) which Amtrak shares with the LIRR, can not handle the higher double-deckers, but can handle 14'-6" (4420 mm) high "split level" cars.

Not all railways were built to standard (generous) 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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