Loading gauge
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A loading gauge is the envelope or contoured shape within which all railroad cars, locomotives, coaches, buses, trucks and other vehicles, must fit. It is dictated by the size of tunnels, height of bridges, the shape, height and position of third rail covers (if the third rail is covered at all) as well as the shape, height and position of railway platforms. Train stops and other signalling equipment must also be cleared and then there is the rack of the rack railways. 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 trains 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.
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[edit] Strictly speaking
- loading gauge is maximum size of rolling stock,
- structure gauge is minimum size of bridges and tunnels,
Therefore, the structure gauge must be larger than the loading gauge.
[edit] Loading gauges of the world
The loading gauge differs around the world. The smallest loading gauge (for a railway of standard gauge track) is that of the London Underground's deep 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 could not predict the future requirements for larger trains and faced 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 and the Chinese loading gauges are also large.
British loading gauge is 9 ft (2743 mm) wide by 11 ft (3353 mm) high on the sides, rising to a 13 ft 6 in (4115 mm) centre. Below platform level (the lower 3 ft 6 in or 914 mm) the vehicle can be no wider than 8 ft 8 in (2642 mm). Some lines, particularly the Hastings Line, had even narrower loading gauges. By contrast the European (Berne) loading gauge is usually 10 ft 2 in (3150 mm) wide by 10 ft 5 in (3175 mm) rising to 14 ft 6 in (4280 mm) in the centre. This is a clearance envelope on a curve of 250 m (820 ft 2.5 in) radius.
[edit] North America
The American loading gauge for freight cars was 15 ft 1 in (4597 mm) high and 10 ft 8 in (3251 mm) wide with 41 ft 3 in (12.573 m) truck (bogie) centers (AAR Plate B and B-1) as well as 15 ft 6 in (4724 mm) high and 10 ft 8 in (3251 mm) wide with 46 ft 3 in (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 ft (134.625 m), see law of sines, examples. Technically, 15 ft 1 in (Plate B) is still the maximum and the circulation of 15 ft 6 in (Plate C) is somewhat restricted, but the extreme frequency of excess-height rolling stock, at first ~18 ft (5486 mm) piggybacks and hicube boxcars then later autoracks, airplane parts cars as well as 20 ft 2 in (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 ft 1 in height, to the Metro-North Railroad and to Amtrak's Northeast Corridor. See also "Additional infrastructure restrictions" in Disadvantages of third rail.
The standard North American passenger car (for trains, not automobiles) is 10 ft 6 in (3200 mm) wide by 14 ft 6 in (4420 mm) high and measures 85 ft 0 in (25.908 m) over coupler faces with 59 ft 6 in (18.136 m) bogie (truck) centers or 86 ft 0 in (26.213 m) over coupler faces with 60 ft 0 in (18.288 m) bogie (truck) centers. In the 1940s and 1950s, the American passenger car loading gauge was increased to a 16 ft 6 in (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 ft 6 in (4420 mm) high "split level" cars. On the Metro-North Railroad and the Long Island Rail Road (including Pennsylvania Station which is owned by Amtrak) the 10" (254 mm) high, above top of rail, safety cover decreases the structure gauge and in turn the loading gauge from top of rail to 11" (280 mm) above top of rail as measured on a 20° curve, which means a radius of 297.94' (87.764 m). These dimensions apply only to North American commuter lines that are used by main line passenger trains and freight trains as well, which is the case for both Metro-North Railroad and the Long Island Rail Road. See also "Additional infrastructure restrictions" in Disadvantages of third rail.
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.
