Editing Dual gauge (section)

===Four rails===
[[File: Cross-section of 4-rail dual-gauge track (standard and metre gauge).png |thumb|left|Four-rail arrangement of standard and metre gauge on the [[Chemin de Fer de la Baie de Somme]]. A 435&nbsp;mm difference in gauge can also be accommodated in a three-rail configuration.]]

In some places, the dimensions of two gauges needing to be collocated are too close to allow a three-rail configuration – for example:
* {{Track gauge|1000mm|lk=on|comma=off}} and {{Track gauge|1067mm|lk=on|comma=off}} (common in Africa, a legacy respectively of French/Belgian and British railway practice)
* {{Track gauge|1000mm|lk=on|comma=off}} and {{Track gauge|914mm|lk=on|comma=off}} (common in South America)
* {{Track gauge|1435mm|lk=on|comma=off}} and {{Track gauge|1524mm|lk=on|comma=off}} (common where [[broad-gauge railway]]s of former [[satellite states]] of the [[Soviet Union]] meet European or Chinese standard gauge).
In such cases, four rails are needed to provide the dual gauge.

Four rails might also be installed because of other engineering or operational factors, even though three rails would suffice: an example is on the [[Chemin de Fer de la Baie de Somme]] (Somme Bay railway), which combines standard and metre gauge – {{convert|435|mm|in|abbr=off|comma=off}} different, well within the parameters for three rails.

Four rails are necessary where the centre-line of rail vehicles on both tracks must be closely aligned with the centre-line of the track in tunnels or other constricted locations. Such configurations, when they revert  back to standard parallel lines as soon as room is available, are termed "[[Gauntlet track|gauntlet track" (US: "gantlet track")]].<ref>{{cite web|url=https://www.trains.com/trn/train-basics/ask-trains/gantlet-gauntlet/ |title=Gantlet vs. gauntlet |author=<!--Not stated--> |date=1 February 2011 |website=[[Trains (magazine)|Trains]] |publisher=[[Kalmbach Media]]|access-date=1 September 2023 }}</ref><ref name=Tratman/>

Four rails must be placed identically on either side of the central axis of dual-gauge [[Railway turntable|turntables]] (and six rails on triple-gauge turntables) so that they match the configuration of the fixed rails leading to and from the turntable, regardless of the direction in which the turntable is facing.

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!colspan="2"| {{larger|''' Gallery: Dual gauge (4 rails)'''}}

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|<br />[[Image: Double écartement CFBS.jpg |border|400x130px|none]]
|<br />[[Image: Rail Baltica Lietuva.jpg |border|400x130px|none]]

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|style="vertical-align: top; width=100"|1435&nbsp;/ 1000&nbsp;mm<br />(4&nbsp;ft 8<span style="vertical-align:super;font-size:x-small">1</span>/<span style="vertical-align:sub;font-size:x-small">2</span>&nbsp;/ 3&nbsp;ft 3<span style="vertical-align:super;font-size:x-small">3</span><span style="vertical-align:sub;font-size:x-small">8</span>&nbsp;in).<br />In the foreground, the narrow gauge diverges via a fixed-rail configuration.{{hair space}}{{efn-la|[[Chemin de Fer de la Baie de Somme]], France, 2007.}}
|style="vertical-align: top; width=100"|1520&nbsp;/ 1435&nbsp;mm<br /> (4&nbsp;ft 11<span style="vertical-align:super;font-size:x-small">27</span>/<span style="vertical-align:sub;font-size:x-small">32</span>&nbsp;/ 4&nbsp;ft 8<span style="vertical-align:super;font-size:x-small">1</span>/<span style="vertical-align:sub;font-size:x-small">2</span>&nbsp;in).<br />The gauges of this track are as follows (numbering the rails from left to right). [[5 ft and 1520 mm gauge railways|1520 mm]] gauge: rails 1 and 3; [[standard gauge|1435 mm gauge]]: rails 2 and 4.{{hair space}}{{efn-la| Newly laid track between [[Mockava]] and [[Šeštokai]], Lithuania, 2013 or earlier.}}
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|<br />[[Image: Haparanda-Tornio rail bridge across the Torne River, looking towards Sweden, 2008.jpg |border|400x130px|none]]
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|style="vertical-align: top; width=100"|1524&nbsp;/ 1435&nbsp;mm<br />(5&nbsp;ft 0&nbsp;/ 4&nbsp;ft 8<span style="vertical-align:super;font-size:x-small">1</span>/<span style="vertical-align:sub;font-size:x-small">2</span>&nbsp;in).<br />A line comprising European standard gauge and "Russian" gauge, a legacy of the former [[satellite state]]s of the [[Soviet Union]]. The 89&nbsp;mm difference between the two gauges is too small to allow a three-rail configuration.{{hair space}}{{efn-la|Torne River  rail bridge between Tornio (Finland) and Haparanda (Sweden, ahead), 2008.}}
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