Introduction
The loading gauge determines the sizes of passenger trains and the size of shipping containers that can be conveyed on a section of railway line and varies across the world and often within a single railway system. Over time there has been a trend towards larger loading gauges and more standardization of gauges with older lines having their structure gauges enhanced by raising bridges, increasing the height of tunnels and making other necessary alterations. Containerisation and a trend towards larger shipping containers has led rail companies to increase structure gauges to compete effectively with road haulage.
The term loading gauge can also refer to a physical structure, sometimes using electronic detectors using light beams on an arm or gantry placed over the exit lines of goods yards or at the entry point to a restricted part of a network. The devices ensure that loads stacked on open or flat wagons stay within the height/shape limits of the line's bridges and tunnels and prevent out-of-gauge rolling stock entering a region with a smaller loading gauge. Compliance with a loading gauge can be checked with a clearance car which in the past were simple wooden frames or physical feelers mounted on rolling stock. More recently lasers are used.
The loading gauge is maximum size of rolling stock which is distinct from the structure gauge which is minimum size of bridges and tunnels which must be larger to allow for engineering tolerances and car motion. The difference between the two is called the clearance. The terms dynamic envelope or kinematic envelope, which include factors such as suspension travel; overhang on curves (at both ends and middle); and lateral motion on the track, are sometimes used in place of loading gauge.
The height of platforms is also a consideration when considering the loading gauge of passenger trains. Where the two are not directly compatible then steps may be required which will increase loading times. Where long carriages are used on a curving platform gaps will occur between the platform and the carriage door causing additional risk. Problems increase where trains of several different loading gauges and train floor heights use the same platform.
The size of load that can be carried on a railway of a particular gauge is also influenced by the design of the rolling-stock. Low deck rolling-stock can sometimes be used to carry taller 9 ft 6 in (2.9 m) shipping containers on lower gauge lines although their low-deck rolling-stock cannot then carry as many containers.
Larger out-of-gauge loads can also sometimes be conveyed 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
- use gauntlet track
- for locomotives that are too heavy, ensure that fuel tanks are nearly empty
Rapid Transit (metro) railways generally have a very small loading gauge. This reduces the cost of tunnel construction. These systems only use their own rolling stock.
Read more about this topic: Loading Gauge
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