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Dual gauge

Dual gauge or mixed gauge railway is a special configuration of railway track, allowing traffic of different axle widths to use the same route alignment. This width, or the distance between the rails on a railway, is referred to as "rail gauge". Generally dual gauge railway consists of three rails, rather than the standard two rails. The two outer rails give the wider gauge, while one of the outer rails, and one inner, allow a narrower gauge. Thus one of the rails is common to all traffic, though this system is not to be confused with a third rail.

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Reasoning

In allowing railway tracks of different gauges to share the same alignment, costs can seemingly be reduced, especially where there are bridges and tunnels. Dual gauge can take the place of two separate sets of tracks (having two rails each), and replace it with one set of track, with 3 rails. This allows one rail less for the stretch of the dual gauge line, however, there are other complications and costs, which usually offset any savings.

One issue is points (US: switches). Complicated arrangements are necessary to ensure traffic of either gauge can safely utilise points. Another issue is wear on the tracks. The common rail will wear much faster than the other two. This complicates maintenance schedules. Signalling may also be complicated somewhat, as all three rails must be connected to track circuits or mechanical arrangements.

Dual gauge turnouts will be complicated, expensive, and suitable for low speeds only.

Configuration

Standard gauge (4 ftin, 1435 mm) and 3 ft 6 in (1067 mm) gauge (as found in Africa) are sufficiently dissimilar to allow 3-rail dual gauge track.

For the dual gauge to work with 3 rails, the difference between the gauges needs to be at least as wide as the foot of the rail, otherwise there is no room for the rail fastening hardware (spikes, clips, and the like). Thus 4 ft 8½ in (1435 mm, standard gauge) and 5 ft 6 in (1676 mm) can be dual gauged without problem, while 4 ft 8½ in (1435 mm) and 5 ft 3 in (1600 mm, Irish gauge) can also be dual-gauged albeit with lighter narrow footed rails, as shown in Victoria, Australia (where the majority of the railways use the 1600 mm gauge). On the other hand, metre gauge (3 ft 3.4 in) and 3 ft 6 in (1067 mm) as found in Africa are too similar to work in 3-rail dual gauge.

If 3-rail dual gauge is not possible, then 4-rail dual gauge may be possible.

Configuration for Africa

1067 mm and 1000 mm gauges found in Africa are too close to allow 3-rail dual gauge. 4-rail dual gauge is required. With a little care, the sleepers for this dual gauge configuration can be made to support triple gauge, including the World Standard Stephenson gauge of 1435 mm at little extra cost.

There is a secondary advantage with such a four rail sleeper. Because the weight of the rail doubles, albeit in four pieces side by side rather than in two pieces of twice the weight, the strength of the track as a whole is increased, not necessarily double, but significantly. By unifying the two types of narrow gauge system, and creating a standard gauge network by happenstance, the possiblity of unifying railways across Africa becomes a bit closer.

If existing African rails are lightweight, say 25 kg/m, and the mainline trunk standard gauge lines need say 50 kg/m rails, then two 25 kg/m rails side by side on concrete sleepers roughly add up to a single 50 kg/m rail. Thus old light rails can be reused.

It is not possible to cover for the small amount of 950 mm gauge in Eritrea.

Example in Africa

On 12 Oct 2004, a proposal was announced to develop a electrified rail link connecting Kenya, Uganda and south Sudan. Even though Kenya and Uganda use 1000mm gauge, while Sudan uses 1067mm, the new project was proposing to use the world standard 1435mm gauge. Fortunately, all three gauges can be supported by the same sleepers, as described above.

Gauge conversion

The complications and difficulties outlined show how important it is to ensure that railway gauges are standardised in the first place, if at all possible. If a railway operator seeks to convert from one gauge to another, then it helps if a dual gauge intermediate step can be done (this has often been actually practised in the past).

If the gauge is to be reduced, then the sleepers can continue to protrude from the path of the rails. If the gauge is to be increased, then the sleepers used for narrow gauge may be too short, and some at least of these 'short' sleepers will have to be replaced with longer ones. Alternatively the rails may be too light for the loads imposed by wider gauge railcars. Such potential problems can rule out dual-gauge as a feasible option. Another issue is affixing the rails to the sleepers (spikes, nails or bolts are used). If existing sleepers are wooden, extra holes can be drilled without problems. If the existing sleepers are concrete, then extra holes are impossible, and the whole sleeper has to be replaced, unless extra boltholes are already allowed for.

During the conversion of the Melbourne to Adelaide line in Australia from 5' 3" to 4' 8&frac12", dual gauge with heavy rails was not possible as the rail footings were too wide. A special gauge convertible sleeper with a chair for the pandrol clip that was reversible allowed 2 week conversion process.

During WW1 and WW2, gauge conversion occurred backwards and forwards between Europe and Russia as the fronts and national borders chopped and changed.

Examples

In Britain, the Great Western Railway initially ran broad gauge traffic. Eventually, after the gauge war , it was decided to regauge the GWR. As the broad gauge was sufficiently dissimilar from standard gauge, and used wooden sleepers, dual gauge was easily introduced for running new standard gauge traffic. The Metropolitan Railway, part of the London Underground system, also started out with dual gauge tracks; however, its current third rail is for electricity supply, not a dual gauge.

In Ireland, dual gauge railway was not used in regauging the Ulster Railway (UR). When it regauged from 6 ft 2 in (1879 mm) to the new standard of 5 ft 3 in (1600 mm), the Ulster Railway merely regauged their dual-track route in two halves. The Dublin & Drogheda Railway (D&DR) meanwhile were regauging from 5 ft 2 in (1574 mm), too similar to the new gauge to allow dual gauge. Dual gauge was used in Derry, by the Port Authority, in an on-street network to transfer goods, in any gauge of traffic, between the city's four stations (two 3 ft narrow gauge, two 5 ft 3 in standard gauge).

In Stuttgart, Germany, the public transit authority uses dual gauge rail on several sections, so that both streetcars, which normally operate amongst auto traffic and are thus smaller, and U-Bahns, which are a bit larger, can operate on the same tracks.

In the Czech Republic, one can find dual gauge rail near Jindřichův Hradec where 1435 mm and 760 mm gauges are placed on the same length of track. Interestingly, the two gauges are used by different railway companies.

In Western Australia, there is a double track dual gauge (3' 6" & 4' 8&frac12") main line from Perth to Northam, about 120km. In Brisbane, Queensland, shorter incidences of dual gauge track (3' 6" & 4' 8&frac12") exist between the rail freight yards at Acacia Ridge and the Port of Brisbane for freight trains. A dual gauge line branches from this track at Park Road Station to run alongside electric suburban narrow gauge CityTrain services over the Merivale Bridge into Platform 1 at Roma St Station . This latter track is for standard gauge interstate CountryLink XPT services to Sydney, which compete with local narrow gauge services to such a degree that often the XPT is forced to terminate south of the border due to congestion.

In Japan, dual gauge is used when the Shinkansen system, which is standard gauge, joins the narrow gauged (1067mm) system, which is national standard in Japan. For example, part of the Ōu line becomes a part of the Akita Shinkansen and was upgraded to dual gauge.

Triple gauge

There have been a few instances of triple gauge break-of-gauge stations.

Because these three triple gauge examples where yards operating at low speed, light rail could be used to space the gauge rails closely together if required. Main line operation at high speeds would be another matter.

Accidents on dual-gauge railways

```On September 9, 2004, a quaint accident happened on a switch in Jindřichův Hradec  where dual gauge railway bifurcates. An express train Junák which was going from Plzeň to Brno derailed here because of switchman's fault. He switched the switch in the direction of narrower track although the express train used standard one. Only the driver of the express train was slightly injured.
```

Complexity of dual-gauge switches

Dual gauge turnouts (aka switches aka points) where both gauges have a choice of routes are quite complicated, with more moving parts that single gauge turnouts. They impose very low speed limits. If dual gauge points are operated and detected by electrical circuits, their reliability will be high.

Where two gauges separate (i.e. each gauge has only one route, as in the picture at right), it is sometimes done without any moving parts.

Separate gauge

If dual gauge turnouts are too slow, or too difficult because the gauges are too similar, then an option is to build two separate lines, one of each gauge, side by side. This choice also depends on the amount of traffic. Dual gauge would continue to be employed at an expensive bridge or tunnel.

Examples include:

• In 2005 a proposed standard gauge line connecting Iran with China via several broad gauge Central asian countries will use a mixture of separate gauge and dual gauge.

Other methods of handling multiple gauges

Other methods of handling multiple gauges include:

• Transporter wagons or transporter trucks, which carry equipment of one gauge on the other's tracks.
• Truck exchange systems, where the railroad car is lifted and the trucks/bogies under it are swapped. This system is not suitable for four-wheel wagons.
• Adjustable gauge equipment, in which the wheel gauge can be widened or narrowed.
• Transshipment; transferring goods or people from one set of railroad cars to another.
• Containerisation.