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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. Though often thought of as a height and width, it is in fact dictated by a number of dimensions and factors: the distance between adjacent tracks, 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.
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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. Though often thought of as a height and width, it is in fact dictated by a number of dimensions and factors: the distance between adjacent tracks, 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, as must 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.
A loading gauge can also be a physical structure- usually an arm or gantry over the track. These are placed over the exit lines of goods yards to ensure that loads stacked on open or flat wagons stayed within the height/shape limits of the line's bridges and tunnels.
Structure gauge vs. loading gauge
- loading gauge is maximum size of rolling stock,
- structure gauge is minimum size of bridges and tunnels,
The structure gauge must be larger than the loading gauge to allow for engineering tolerances and car motion. The difference between the two is called the clearance.
Platform height vs. Train floor height
- Railway platform height and Train floor height is where the structure gauge and the loading gauge meet. This variable is not frequently described with loading and structure gauges but becomes a critical factor in passenger safety and train line efficiency. Steps slow the boarding process. Level entry is faster. But where loading gauge and structure gauge differences create gaps, customers have to be notified to "mind the gap" or the steps. Problems are greatly compounded where trains of several different loading gauges and train floor heights use the same platform.
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. In mainland Europe, lines tend to conform to the slightly larger Berne gauge and loading gauges in the North America tend to be larger still. The Russian (including Finnish and ex-Soviet) and the Chinese loading gauges are also very large, whereas the Scandinavian come in between.
Britain
British loading gauge is wide by high on the sides, rising to a centre. Below platform level (the lower the vehicle can be no wider than . Some lines, particularly the Hastings Line, had even narrower loading gauges. By contrast the European (Berne) loading gauge is usually wide by rising to in the centre. This is a clearance envelope on a curve of radius.
British loading gauges currently use a classification system prefixed with 'W'. This ranges, in height at least, from W6a to W12. W6a, formerly British Rail W6, is available over the majority of the British rail network. A strategy was adopted in 2004 to guide enhancements of loading gauges.
North America
The American loading gauge for freight cars on the North American rail network is generally based on standards set by the Association of American Railroads (AAR) (Mechanical Division) . The most widespread standards are AAR Plate B and AAR Plate C, but higher loading gauges have been introduced on selected routes to accommodate rolling stock that make better economic use of the network, such as auto carriers and double-stack container cars.
Height Clearance North America: Plate B - . Plate C - . Plate E - . Plate F - . Plate H - (Double Stacks). (source - April 2001 Official Railway Equipment Register). Plate J - ? (e.g. Long flatcars ) Plate K - ? (e.g. Autorack road vehicles on trains).
Freight
AAR Plate B allows cars high and wide with truck (bogie) centers. When the distance between trucks exceeds 41 ft 3 in, the width is decreased according to graph AAR Plate B-1. AAR Plate C allows cars high and wide with truck (bogie) centers. When the distance between trucks exceeds 46 ft 3 in, the width is decreased according to graph AAR Plate C-1 .
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 frequency of excess-height rolling stock, at first ~ piggybacks and hicube boxcars then later autoracks, airplane parts cars as well as high double-stacked containers in container well cars, means that many, but not all, lines are now designed for a higher loading gauge. The width of these extra height cars is covered by Plate C-1 . However, additional 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.
Passenger
The standard North American passenger railcar is wide by high and measures over coupler faces with bogie (truck) centers or over coupler faces with bogie (truck) centers. In the 1940s and 1950s, the American passenger car loading gauge was increased to a height in the West to accommodate dome cars and later Superliners and other double-decker trains. Amtrak's Northeast Corridor, especially Pennsylvania Station which Amtrak owns, but shares with the LIRR, can not handle the higher double-deckers, but can handle high "split level" cars.
On the Metro-North Railroad and the Long Island Rail Road (including Pennsylvania Station) the high, above top of rail, safety cover decreases the structure gauge and in turn the loading gauge from top of rail to above top of rail as measured on a 20° curve, which means a radius of . These dimensions apply only to third rail electrified North American commuter lines that are used by main line passenger trains and freight trains as well. See also "Additional infrastructure restrictions" in Disadvantages of third rail. The loading gauge of the Mount Royal Tunnel in Montreal is also restricted .
General
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.
International containers
The size of International Containers was determined ( by 8ft by (n×10) ft) in the 1950s to be suitable for a reasonably high proportion of railway systems. Some work had to be done to raise bridges and tunnels, such as the Rhyndaston Tunnel on the Tasmanian Government Railways. As the height of containers increased in the 2000s, further work had to be done.
Loading gauge detectors
To help prevent out-of-gauge rolling stock entering a region with a smaller loading gauge, electronic detectors using light beams are used to detect such rolling stock.
Accidents
Accidents inevitably occur when an out-of-gauge load or vehicle is present on a train, and suitable precautions are not observed.
Out-of-gauge trains
From time-to-time out-of-gauge loads need to be carried, and these can proceed with care, by taking one or more of the following measures:
- operate at low speed, especially at places with limited clearance such as platforms
- crossover from track with inadequate clearance to another track with greater clearance, even if there is no signalling to allow this.
- prevent operation of other trains on adjacent tracks.
- use refuge loops to allow trains to operate on other tracks.
- use special rolling stock that manipulate the load up and down or left and right to clear obstacles.
- remove (and later replace) obstacles.
- for locomotives that are too heavy, ensure that fuel tanks are nearly empty.
See also
External links
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