Editing Dual gauge (section)

==Background==
{{Hatnote|A comprehensive list of rail gauges around the world is in the article about [[Track gauge]].}}

The rail gauge is the most fundamental specification of a railway. [[Rail tracks]] and [[Wheelset (rail transport)|wheelsets]] are built within [[engineering tolerance]]s that allow optimum lateral movement of the wheelsets between the rails. Pairs of rails that become too wide or narrow in gauge will cause derailments, especially if in excess of normal gauge-widening on curves.<ref name=Tratman>{{cite book |last=Tratman |first=E.E. Russell |date=1908 |url= https://archive.org/details/railwaytracktrac01trat/page/n9/mode/2up |title=Railway track and track work |publisher=The Engineering News Publishing Co. |edition=3 |location=New York|access-date=8 November 2022 |page=402}}</ref>{{refn|group=note|Two-rail track on curves is re-gauged when the gauge is 20 mm over (16 mm on narrow gauge). It is sometimes done by relocating the low (inner) rail to save "spike-killing" the sleeper at the outer end, where lateral loads are greater.  In other cases, the sleeper may be slid through by about 70 mm to provide "new" timber for spiking.<ref>{{cite book |date=1988 |title=Track Maintenance Guide |publisher=Australian National Railways Commission |location=Adelaide |page=section 12.2}}</ref> On dual-gauge turnouts, tolerances are small: only {{convert|8|mm|in|abbr=off}} over gauge and {{convert|2|mm|in|abbr=off}} under.<ref>{{cite web |url=https://extranet.artc.com.au/docs/eng/track-civil/workinstruct/pc/ETN-03-01.pdf |title=50kg dual gauge turnouts and gauge separations – construction and maintenance manual |author=<!--Not stated-->|date=2008 |website=Australian Rail Track Corporation |access-date=7 November 2022 }}</ref>}}

Given the requirement for gauge to be within very tight limits, when the designed distance between the pair of wheels on a wheelset differs even slightly from that of others on a railway, track must be built to two specific gauges. That is achieved in a variety of ways: most commonly by adding a third rail, more rarely by adding another pair of rails; and rarer still, when three gauges are present, by four rails.