Editing Loading gauge (section)

==Overview==
[[File:London Underground subsurface and tube trains.jpg|thumb|right|The [[London Underground]] utilises differing loading gauges: a [[Metropolitan line]] [[London Underground A60 and A62 Stock|A Stock]] sub-surface train ''(left)'' passes a [[Piccadilly line]] [[London Underground 1973 Stock|1973 Stock]] tube train ''(right)''.]]
The loading gauge restricts the size of passenger coaches, goods wagons (freight cars) and [[intermodal container|shipping containers]] that can travel on a section of railway track. It 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; some older lines have had their [[structure gauge]]s enhanced by raising bridges, increasing the height and width of tunnels and making other necessary alterations. [[Containerisation]] and a trend towards larger [[intermodal container|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 [[electric eye|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 stretch of line with a smaller loading gauge. Compliance with a loading gauge can be checked with a [[clearance car]]. In the past, these were simple wooden frames or physical feelers mounted on rolling stock. More recently, [[laser]] beams are used.

The loading gauge is the maximum size of rolling stock. It is distinct from the [[structure gauge|minimum structure gauge]], which sets limits to the size of bridges and tunnels on the line, allowing for [[engineering tolerance]]s and the motion of rail vehicles. The difference between the two is called the [[engineering tolerance|clearance]]. The terms "dynamic [[envelope (motion)|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.{{Citation needed|date=May 2009}}

The [[railway platform height]] is also a consideration for the loading gauge of passenger trains. Where the two are not directly compatible, stairs may be required, which will increase [[Dwell time (transportation)|loading times]]. Where long carriages are used at a curved platform, there will be [[platform gap|gaps between the platform and the carriage door]], causing risk. Problems increase where trains of several different loading gauges and train floor heights use (or even must pass without stopping at) 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 {{convert|9|ft|6|in|m|1|abbr=on}} shipping containers on lower gauge lines although their low-deck rolling stock cannot then carry as many containers.

[[Rapid transit]] (metro) railways generally have a very small loading gauge, which reduces the cost of tunnel construction. These systems only use their own specialised rolling stock.

=== Out of gauge ===
Larger ''out-of-gauge'' loads can also sometimes be conveyed by taking one or more of the following measures:
* Operate at low speed, especially in places with limited clearance, such as platforms.
* Cross over from a 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 of [[Schnabel car]]s (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]] to shift the train to side or center.
* For locomotives that are too heavy, ensure that fuel tanks are nearly empty.
* Turn off power in overhead wiring or in the third rail (use diesel locomotive)
* Permanently adapt a certain route to larger gauge if there is repeated need for such trains.