Dual gauge: Difference between revisions

Dual 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.

In allowing railway tracks of different gauges to share the same alignment, costs can seemingly be reduced. 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.

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 "spikes" or "nails". Thus 4 ft 8½ in (1435 mm, standard gauge) and 5 ft 6 in (1676.4 mm) can be dual gauged without problem, while 4 ft 8½ in and 5 ft 3 in (1600 mm, Irish gauge) can also be dual-gauged, 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.

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 (US: ties, the cross-sectional bars supporting the rails) 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 a proportion 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.

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 the electricity supply, not a dual guage.

In Ireland, dual gauge railway was not used in regauging - the Ulster Railway (UR), who regauged from 6 ft 2 in (1879.6 mm) to the new standard of 5 ft 3 in (1600 mm), merely regauged their dual-track route in two halves. The Dublin & Drogheda Railway (D&DR) meanwhile were regauging from 5 ft 2 in (1574.8 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.

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