Editing Track gauge (section)

==Dual gauge==
{{Main|Dual gauge}}
[[File:Cross-section of 4-rail dual-gauge track (standard and metre gauge).png|thumb|right|Cross-section of 4-rail dual-gauge track (standard and metre gauge/ narrow gauge) (click to enlarge)]]
[[File:Cross-section of Australian dual-gauge railway track.tif|thumb|right|Cross-section of Australian dual-gauge track – {{Track gauge|1600mm|comma=off}} and {{Track gauge|1435mm|comma=off}} gauges (click to enlarge)]]
[[File:Mixed gauge Sassari.jpg|thumb|Mixed gauge track at Sassari, Sardinia: {{Track gauge|1435mm|allk=on}} and {{Track gauge|950mm|lk=on}}]]

When individual railway companies have chosen different gauges and have needed to share a route where space on the ground is limited, [[Dual gauge|mixed gauge]] (or dual gauge) track, in which three (sometimes four) rails are supported in the same track structure, can be necessary. The most frequent need for such track was at the approaches to city terminals or at [[Break of gauge|break-of-gauge]] stations.

Tracks of multiple gauges involve considerable costs in construction (including signalling work) and complexities in track maintenance, and may require some speed restrictions. They are therefore built only when absolutely necessary. If the difference between the two gauges is large enough – for example between {{Track gauge|sg|allk=on}} and {{Track gauge|3ft6in|lk=on}} – three-rail dual-gauge is possible, but if not – for example between {{Track gauge|3ft6in}} and {{Track gauge|1000mm|allk=on}} – four rails must be used. Dual-gauge rail lines occur (or have occurred) in Argentina, Australia, Brazil, Japan, North Korea, Spain, Switzerland, Tunisia and Vietnam.

[[File:Mixed gauge track Didcot.JPG|thumb|Reconstructed [[Dual gauge|mixed-gauge]], {{Track gauge|1435mm|allk=on}} / {{Track gauge|7ft0.25in|lk=on}} track at [[Didcot Railway Museum]], England]]
On the GWR, there was an extended period between political intervention in 1846 that prevented major expansion of its {{Track gauge|7ft0.25in|lk=on}} [[broad gauge]]<ref group = note>The Act of Parliament did not prohibit expansion of the existing broad gauge system, but it had the indirect and delayed effect of forcing conformity with the "standard" gauge eventually.</ref> and the final [[Track gauge conversion|gauge conversion]] to standard gauge in 1892. During this period, many locations practicality required mixed gauge operation, and in station areas the track configuration was extremely complex. This was compounded by the common rail having to be at the platform side in stations; therefore, in many cases, standard-gauge trains needed to be switched from one side of the track to the other at the approach. A special fixed point arrangement was devised for the purpose, where the track layout was simple enough.<ref group="note">{{cite book |first1=S. C. |last1=Jenkins |name-list-style=and |first2=R. C. |last2=Langley |title=The West Cornwall Railway |publisher=The Oakwood Press |location=Usk |year=2002 |isbn=0853615896 |mode=cs2}}, gives an illustration and description on page 66.</ref>

In some cases, mixed gauge trains were operated with wagons of both gauges. For example, MacDermot<ref name = macdermot2>E. T. MacDermot (1931), ''History of the Great Western Railway'', vol. II: 1863–1921, London: Great Western Railway, p. 316</ref> wrote:

<blockquote>In November 1871 a novelty in the shape of a ''mixed-gauge'' goods train was introduced between Truro and Penzance. It was worked by a narrow-gauge engine, and behind the narrow-gauge trucks came a broad-gauge match-truck with wide buffers and sliding shackles, followed by the broad-gauge trucks. Such trains continued to run in West Cornwall until the abolition of the Broad Gauge; they had to stop or come down to walking pace at all stations where fixed points existed and the narrow portion side-stepped to right or left.</blockquote>