In railway engineering, "gauge" is the transverse distance between the inner surfaces of the heads of two rails, which for the vast majority of railway lines is the number of rails in place. However, it is sometimes necessary for track to carry railway vehicles with wheels matched to two different gauges. Such track is described as dual gauge – achieved either by addition of a third rail, if it will fit, or by two additional rails. Dual-gauge tracks are more expensive to configure with signals and sidings, and to maintain, than two separate single-gauge tracks. It is therefore usual to build dual-gauge or other multi-gauge tracks only when necessitated by lack of space or when tracks of two different gauges meet in marshalling yards or passenger stations. Dual-gauge tracks are by far the most common configuration, but triple-gauge tracks have been built in some situations.
The rail gauge is the most fundamental specification of a railway. Rail tracks and wheelsets are built within engineering tolerances 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.[1]
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.
Dual-gauge track can consist of three rails, sharing one "common" rail; or four rails, with the rails of the narrower gauge lying between those of the broader gauge. In the three-rail configuration, wear and tear of the common rail is greater than with the two other outer rails. In dual gauge lines, turnouts (railroad switches) are more complex than in single-gauge track, and trains must be safely signalled on both of the gauges. Track circuits and mechanical interlocking must also operate on both gauges.[2]
Multi-gauge track is very often associated with a break-of-gauge station, where rail vehicles or vehicle contents are transferred from one gauge to another. A break of gauge causes delay and increases congestion, especially on single-track lines. Essentially, two trains are required to do what a single train would normally accomplish. When traffic passes mainly in one direction, full wagons taken to the border have to be returned as empties, and a train of empty wagons has to be brought to the break of gauge from the other side to fetch the cargo. Congestion is also caused by unloading and reloading.[3] [4] The problem is worsened when there is a disparity between the capacity of locomotives and vehicles on the two gauges: typically, one broad-gauge trainload needs three narrow-gauge trains to carry.[5]
Constructing dual-gauge track with three rails is possible when the two adjacent rails can be separated at the base by at least the space required by rail fastening hardware such as spikes and or rail clips – typically 40abbr=offNaNabbr=off. If the two gauges are closer than that, four rails must be used. Depending on the rail fasteners used and the weight of rails (heavy rails are bigger), the practicable difference between the two gauges is in the range 145abbr=offNaNabbr=off to 200abbr=offNaNabbr=off.[6]
In some places, the dimensions of two gauges needing to be collocated are too close to allow a three-rail configuration – for example:
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 – 435abbr=offNaNabbr=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" (US: "gantlet track").[7] [1]
Four rails must be placed identically on either side of the central axis of dual-gauge 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.
In rare situations, three different gauges may converge on to a rail yard and triple-gauge track is needed to meet the operational needs of the break-of-gauge station – most commonly where there is insufficient space to do otherwise. Construction and operation of triple-gauge track and its signalling, however, involves immense cost and disruption, and is undertaken when no other alternative is available.[8]
The following table shows localities where triple gauge has been necessary.
| Place | Narrowest gauge | Middle gauge | Broadest gauge | Notes |
|---|---|---|---|---|
| Volos, Greece | gauge closed in the 1970s, gauge closed in 1998 | |||
| Jenbach, Austria | Still in use | |||
| Montreux, Switzerland | ||||
| Capolago, Switzerland | Metre gauge line closed in 1950 | |||
| Växjö, Sweden | Until 1974 or later[9] | |||
| Vynohradiv, Ukraine | Standard and broad gauge track is track plait between Halmeu, Romania and Čierna nad Tisou, Slovakia, near Ukraine. The narrow-gauge track belongs to Borzhava narrow-gauge railway. All still in use. | |||
| Latour-de-Carol, France | Still in use | |||
| Hendaye, France | ||||
| 1968–1983 (Some has been reconstructed at the National Railway Museum, Port Adelaide.)[10] | ||||
| Peterborough, South Australia | 1968–1983. (Some has been preserved at the Steamtown Heritage Rail Centre in Peterborough.)[11] | |||
| Toronto, Canada | Reported as impending in 1872.[12] Subsequently converted to gauge. | |||
Three gauges are the maximum found on operating railway lines and in railway yards, but some rolling stock manufacturers collocate more than three lines in their works, depending on the particular gauges of their customers.
Transfer of freight and passengers between different gauges does not necessarily involve dual-gauge track: there may simply be two tracks that approach either side of a platform without overlapping. In Australia, 13 break-of-gauge stations existed by 1945 as a result of longstanding interstate rivalries: three different gauges had persisted since the 1850s and the five mainland state capitals were not linked by standard gauge until 1995.[13] Huge costs and long delays were imposed by trans-shipment of freight at break-of-gauge stations, whether manually, by gantry crane or by wheelset or bogie exchange.[4] During World War II, breaks of gauge in Australia added immense difficulty to the war effort by needing extra locomotives and rolling stock, and more than 1600 service personnel and a large pool of civilians, at transfer points for an annual average transfer of about 1.8 million tonnes of freight.[14]
To cost and inefficiency was added, in the case of passengers, considerable inconvenience. In 1896, at Albury station on the Sydney–Melbourne railway, famed American writer Samuel Clemens (Mark Twain) had to change trains in the middle of a "biting-cold" night in 1896 and there formed his pungent view of "the paralysis of intellect that gave that idea birth".[15]
In some locations, an alternative to building long lengths of dual-gauge track has been to change the wheels on rolling stock, either by dropping and changing wheelsets from four-wheeled vehicles or exchanging bogies (US: trucks) under eight-wheeled vehicles. With this arrangement, a short length of dual-gauge track is only needed within the facility. A benefit is that the contents of fully loaded cars are not disturbed. The scheme was first adopted on the French–Spanish border and in Poland. It introduces delay into transit times compared with dual-gauge operation, but is much quicker than trans-shipping: when introduced in 1962 in Melbourne, Australia, on the route between Sydney and Adelaide, the freight handling time per train dropped from five days to less than two. The process involved disconnecting the brake rigging and bogie centre pins have to be disconnected before the vehicle is lifted and new bogies are wheeled underneath.[16]
In Europe, a similar principle embodies low-profile, small-wheeled transporter wagons, which carry vehicles built for one gauge on a line with a different gauge. A variant is the rollbock (Rollböcke in German), used under two-axle standard-gauge vehicles: each wheelset is carried on a small four-wheeled narrow-gauge trolley.[17] The entire train is converted in minutes at a slow walking pace, each rollbock being automatically matched to its wheelset from underneath.
A further variant is "train on train", in which an entire narrow-gauge train is carried on standard-gauge flatcars on which continuous rail has been fitted.[18] [19] [20]
Differences in gauge are also accommodated by gauge-adjustable wheelsets, which were installed under some passenger vehicles on international links between Spain and France, Sweden and Finland, Poland and Lithuania, and Poland and Ukraine. In Spain, change-over facilities are extensive, since although 1668abbr=offNaNabbr=off track predominates, and high-speed lines are laid to standard gauge, there are many lines with narrower gauges (1000abbr=offNaNabbr=off and others).[21]
In Victoria, there are sections of and dual-gauge track between Southern Cross station and West Footscray, Sunshine and Newport, Albion and Jacana, North Geelong and Gheringhap, Maryborough and Dunolly, and in various goods yards and industrial sidings. Until 2008, there was a dual-gauge line between Wodonga and Bandiana.At Albury railway station, New South Wales, a and dual-gauge line was in place until 2011. A dual-gauge line was within Tocumwal railway station until 1988, when the standard gauge component was put out of use.
In 1900, in South Australia, a three-rail dual-gauge system was proposed in order to avoid a break of gauge. However, designing turnouts was considered to be difficult due to the difference of only 165mm between the and the broad gauge. After twenty years of discord, the proposal was abandoned.[22] [23] [24] Much later, the South Australian Railways successfully adopted dual-gauge turnouts.[25]
In Western Australia, and of double-track dual-gauge extends for 120km (80miles) of the main line from East Perth to Northam. Dual-gauge track is also used from the triangle at Woodbridge to Cockburn Junction, then to Kwinana on one branch and North Fremantle on the other. The signalling system detects the gauge of the approaching train and puts the signals to stop if the route is set for the wrong gauge.
In Queensland, there is a section of and dual-gauge track between the rail freight yards at Acacia Ridge and Park Road station, which is utilised by both passenger and freight trains. Freight trains to the Port of Brisbane utilise the dual gauge Fisherman Islands line that runs parallel to the Cleveland railway line from Park Road to Lindum. Passenger trains use the dual-gauge section of the Beenleigh railway line running parallel to the electric suburban narrow gauge of the Queensland Rail city network over the Merivale Bridge into platforms 2 and 3 at Roma Street Station. This is used by standard gauge interstate New South Wales TrainLink XPT services to Sydney. In 2012, a dual-gauge line was installed between Acacia Ridge and Bromelton to serve a new freight hub at Bromelton.
The 1700abbr=offNaNabbr=off long Inland Railway, under construction in 2022, will have about 300abbr=offNaNabbr=off of dual gauge.
Tram tracks in Brussels once combined lines for inter-urban trams and lines for urban trams in a three-rail layout. In 1991, the interurban trams went out of service and then the network used only standard-gauge track.
The new port of Kribi may serve 1000mm gauge bauxite traffic as well as 1435mm gauge iron ore traffic.
In the Czech Republic, there is 2 km of dual gauge and track near Jindřichův Hradec. In 1985, its original four rails were converted to three rails. In 2004, in Jindřichův Hradec at a switch where a dual gauge railway bifurcates, a Junák express from Plzeň to Brno derailed due to a signalling error. The standard gauge train had been switched on to the narrow gauge track.
The Chemin de Fer de la Baie de Somme in France is dual gauge between Noyelles-sur-Mer and Saint-Valery-sur-Somme. The line has four rails with metre gauge laid within standard gauge. There are some dual-gauge (standard and Iberian) sidings at Cerbère on the Spanish border.
In the 1970s, the Stuttgarter Straßenbahnen tram lines underwent a gauge conversion from gauge to standard gauge. This was part of an upgrade to the Stuttgart Stadtbahn. In 1981, and dual-gauge track was constructed so that new DT-8 Stadtbahn cars and old trams could share the network. In 2008, a further gauge conversion was completed. The Stuttgart Straßenbahn Museum operates gauge trams on weekends and special occasions.In Krefeld on Ostwall, tram lines are dual gauge so that standard Rheinbahn U76 Stadtbahn cars and gauge trams may share the lines. At the north end of the route, at the junction with Rheinstraße, the trams reverse. There, the standard gauge line ends, while the metre gauge lines continue. At the Hauptbahnhof, on Oppumer Straße, dual gauge track continues. At the ends of Oppumer Straße, the two tracks diverge.
In Mülheim there is a similar situation. The Duisburg tram line 901 meets the local line 102. The tram system in Duisburg uses gauge track while the tram route from Witten to Mülheim uses gauge tracks. Two lines share a tunnel section between the Mülheim (Ruhr) Hauptbahnhof and Schloss Broich then diverge at street level.
The tram network between Werne to Bad Honnef is large with various operators and gauges. The trams in Wuppertal used gauge track on eastwest lines and gauge track on northsouth lines. Trams in Duisburg used gauge track on lines south of the Ruhr and gauge tracks on lines north of the Ruhr. The north lines closed in the 1960s and 1970s. Duisburg's three routes were converted to gauge track.
Ghana is converting its narrow gauge to standard gauge, and is installing dual-gauge sleepers as an intermediate stage.In 1942 and 1943 in Java, under Japanese military occupation, conversion from took place to on the BrumbungKedungjatiGundih main line and the KedungjatiAmbarawa branch line.
Until the 1970s, a short section of dual gauge and line existed in North Sumatra on a joint line of the Deli Railway and the Atjeh Tram.
Some sugar mill railways in Java have dual-gauge sections.
The Potenza Avigliano Lucania line in Italy is a dual gauge rail with and tracks.
In Japan, the national standard is narrow gauge. Dual gauge is used where the Shinkansen (bullet train) lines join the main network. For example, part of the Ōu Main Line became part of the Akita Shinkansen and was converted to dual gauge in a limited section. The longest (82.1km (51miles)) dual gauge section in Japan is near, and in, the Seikan Tunnel. Sections of the Hakone Tozan Line are among a number of other dual-gauge lines.
Mexico previously had and dual gauge track.
The first railway lines in the Netherlands were constructed with a track gauge of . For the 1939 centennial celebration, an exact replica of the country's first locomotive "De Arend" was built using the original blueprints. Since 1953, the locomotive is housed at the Dutch National Railway Museum, where in recent years, a dual-gauge track has been constructed in the rail yard, allowing for the locomotive to drive back and forth on special occasions.[26] 
In Poland, there is 3abbr=offNaNabbr=off of and dual-gauge track in the Greater Poland Voivodeship, linking Pleszew with a nearby mainline station. It is served by narrow-gauge passenger trains and standard-gauge freight trains.
Between 2008 and 2012, a 2km (01miles) dual-gauge cross-border track was rebuilt between Khasan, Russia, and Rajin, North Korea; its gauges were the Russian and Korean .[27] Similar arrangements exist on the approach to Kaliningrad, where track extends from the Polish border with some sections of dual gauge.
In Spain, there is 21.7km (13.5miles) of dual gauge in the AVE line from Zaragoza to Huesca, usable for both standard-gauge high-speed trains and Spanish network trains. Some dual-gauge sidings are at Port Bou on the French border. In 2009, Adif called for tenders for the installation of a third rail for standard-gauge trains on the 22km (14miles) between Castellbisbal and the Can Tunis freight terminal in Barcelona.[28] 
The bridges at the borders of Sweden and Finland, between Haparanda and Tornio have 2km (01miles) of dual gauge, and track. At each end of the dual-gauge section are yards with standard and Finnish gauge areas to allow for trans-shipment. Four rails are used because the gauges are close and the bridge structure is wider than normal to allow for the offset from the centreline of each gauge. A Rafil gauge changer is at the Tornio yard. Between Västervik and Jenny, Sweden, there is a and dual-gauge line and dual-gauge track in the Västervik station area.
In Switzerland, dual-gauge track and track exists between Lucerne and Horw of the Zentralbahn, between Niederbipp and Oberbipp of the Oberaargau-Jura Railways and between Chur and Domat/Ems of RhB. All three allow narrow-gauge passenger trains and standard-gauge freight trains to operate. A ″non-stop″ scheme (albeit with a pause to change locomotives) on the Montreux-Interlaken route was inaugurated in December 2022.[29] The former Zollikofen-Worblaufen-Deisswil dual gauge was cut back to Papiermühle when the factory in Deisswil closed.
The railway tracks between the border with Slovakia at Solomonovo and the border with Romania at Nevetlenfolu are dual-gauge, and . This is in part due to the Austro-Hungarian heritage of the Zakarpattia Oblast, but also has military and political importance, as it allows standard-gauge trains to run between Romania and Slovakia by bypassing Hungary, otherwise impossible due to the break-of -gauge between the Russian-gauge network of Ukraine and standard-gauge networks of Romania and Slovakia.
The Great Western Railway in Britain was originally built to a broad gauge of 2134 mm (7 ft 0 in), subsequently widened to 2140 mm (7 ft 0 in). After a "gauge war", the gauge was converted to . A dual-gauge system was easily installed as the gauges were well separated and the line had wooden sleepers. A short section of broad and standard gauge is at the Great Western Society site at Didcot.The port authority in Derry, Northern Ireland, used a dual-gauge line in a street-level network to transfer freight. Two of the city's stations were on a narrow gauge. The other two city stations were on broad gauge.
The Fairbourne Railway in Gwynedd, Wales, has a section of dual gauge track from Fairbourne station to Car Park crossing installed in the late 2010s to allow visiting gauge trains to run on part of the line, which was converted to gauge in 1986, usually as part of special events.
In Los Angeles, the Los Angeles Railway and Pacific Electric Railway ran on dual gauge track on some downtown streets.From 1880 to 1902, the Burlington, Cedar Rapids and Northern Railway (standard gauge) and the Burlington and Northwestern Railway (narrow gauge) shared a dual-gauge mainline from Burlington, Iowa, to Mediapolis, 23km (14miles) to the north.
The early operational years of the State Belt Railroad in San Francisco featured dual-gauge tracks to accommodate regional railroads of the time, which interchanged via ferry.
Until 1941, the Colorado and Southern Railway used both standard-gauge and narrow-gauge tracks, and had a dual-gauge line between Denver and Golden, Colorado.
Until the 1960s, the Denver and Rio Grande Western Railroad's Alamosa–Durango Line from Alamosa, Colorado, to Antonito was dual-gauge (and).
Previously, in its yard at Mount Union, Pennsylvania, the East Broad Top Railroad and Coal Company used dual-gauge tracks.
. Tales from a Railway Odyssey. Keith Smith (engineer). 114.