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5 ft 6 in / 1,676 mm, a broad gauge, is the track gauge used in India, Pakistan, western Bangladesh, Sri Lanka, Argentina, Chile, and on the BART (Bay Area Rapid Transit) in the San Francisco Bay Area.
In North America, it is called Provincial, Portland, or Texas gauge. In Argentina, it is known as "trocha ancha" (Spanish for broad gauge). In the Indian Subcontinent it is simply known as "broad gauge". Elsewhere it is known as Indian gauge. It is the widest gauge in regular passenger use anywhere in the world.
In India, the initial freight railway lines were built using standard gauge. In the 1850s, the Great Indian Peninsula Railway adopted the gauge of 1,676 mm (5 ft 6 in) for the first passenger railway in India between Bori Bunder and Thane. This was then adopted as the standard for the nationwide network.
Rail transport in India today is predominantly on 1,676 mm (5 ft 6 in) broad gauge with more than 120,000 km of tracks. Most of the metre gauge and narrow gauge railways have been converted to broad gauge. Small stretches of the network that remain on metre and narrow gauges are also being converted to broad gauge. Rapid transit lines, are mostly on standard gauge, although some initial lines use 1,676 mm (5 ft 6 in) broad gauge.
Bangladesh Railways uses a mix of 1,676 mm (5 ft 6 in) broad gauge and metre gauge. The broad gauge network is primarily located to the west of the Jamuna River, while the metre gauge network is primarily located to its east. The Jamuna Bridge is a mixed-use bridge that contains a dual gauge connection across the river linking both networks.
In Nepal all services currently operate on 1,676 mm (5 ft 6 in) broad gauge only.
In Pakistan, all services currently operate on 1,676 mm (5 ft 6 in) broad gauge only.
In Sri Lanka, all services currently operate on 1,676 mm (5 ft 6 in) broad gauge only.
The 1,676 mm (5 ft 6 in) broad gauge was actually first used in Scotland for two short, isolated lines, the Dundee and Arbroath Railway (1836-1847) and the Arbroath and Forfar Railway (1838-). Both the lines were subsequently converted to standard gauge.
The Iberian-gauge railways, that service much of Spain and Portugal, have a track gauge of 1,668 mm (5 ft 5 21⁄32 in), just 8 mm (5⁄16 in) different from 1,676 mm (5 ft 6 in). Used rolling stock from Iberia has been employed on broad-gauge lines in Argentina and Chile.
Canada became the first British colony, in the 1850s, to use 1,676 mm (5 ft 6 in) broad gauge. It was known as the "Provincial gauge" in Canada.
The Grand Trunk Railway which operated in several Canadian provinces (Quebec and Ontario) and American states (Connecticut, Maine, Massachusetts, New Hampshire, and Vermont) used it, but was changed to standard gauge in 1873. The Grand Trunk Railway operated from headquarters in Montreal, Quebec, although corporate headquarters were in London, England. The St. Lawrence and Atlantic Railroad which operated in Quebec, Vermont, New Hampshire and Maine also used it but was converted in 1873.
There is a longstanding rumour that the Provincial gauge was selected specifically to create a break-of-gauge with US railways, the War of 1812 still being a fresh memory. However, there is little supporting evidence for this, and this story appears to be traced to a single claim from the late 1800s.
The Bay Area Rapid Transit system is the only operating railroad in the United States to use 1,676 mm (5 ft 6 in) broad gauge, with 109 miles (175 km) of double tracked routes. The original engineers for the system had background in aerospace (rather than railroads) and intended to make a state-of-the-art system for other municipalities to emulate. The use of 1,676 mm (5 ft 6 in) broad gauge rails was one of many unconventional design elements included in its design which, in addition to its unusual gauge, also uses flat-edge rail, rather than typical rail that angles slightly inward. This has complicated maintenance of the system, as it requires custom wheelsets, brake systems, and track maintenance vehicles.
The New Orleans, Opelousas and Great Western Railroad (NOO&GW) used 1,676 mm (5 ft 6 in) broad gauge until 1872, and the Texas and New Orleans Railroad used 1,676 mm (5 ft 6 in) broad gauge, ("Texas gauge") until 1876. The Grand Trunk Railway predecessor St. Lawrence and Atlantic Railroad which operated in Quebec, Vermont, New Hampshire and Maine also used 1,676 mm (5 ft 6 in) broad gauge ("Canadian gauge", "Provincial gauge" or "Portland gauge") but was converted in 1873. Several Maine railroads connected to the Grand Trunk Railway shared its "Portland Gauge". The Androscoggin and Kennebec Railroad and the Buckfield Branch Railroad were later consolidated as the Maine Central Railroad which converted to standard gauge in 1871. The only electric streetcar system in the U.S. to use this gauge was that of Fairfield, Maine. 
The national railway network is predominantly on 1,676 mm (5 ft 6 in) broad gauge.
Most installations of 1,676 mm (5 ft 6 in) broad gauge railways are in the south of the country.
The Iberian gauge (1,668 mm or 5 ft 5 21⁄32 in) is closely similar, with only 8 mm (5⁄16 in) difference, and allows compatibility with the rolling stock. For example, in recent years Chile and Argentina have bought second hand Spanish/Portuguese Iberian-gauge rolling stock. 1,668 mm trains can run on 1,676 mm gauge without adaptation, but for better stability in high-speed running a wheelset replacement may be required (for example, Russian-Finnish train Allegro has gauge 1,522 mm, intermediate between Russian 1,520 mm and Finnish 1,524 mm). Backward compatibility—1,676 mm trains on 1,668 mm gauge—is possible, but no examples and data exist. Due to the narrower gauge, a strong wear of wheelsets may occur without replacement.
|Argentina||San Martín Railway||operating|
|Argentina||Mitre Railway||except Tren de la Costa in standard gauge; operating|
|Argentina||Roca Railway||except La Trochita, Central Chubut Railway and Ramal Ferro Industrial Río Turbio in 750 mm (2 ft 5 1⁄2 in) gauge; operating|
|Bangladesh||Bangladesh Railway||682 km (424 mi)||operating|
|Chile||*Empresa de los Ferrocarriles del Estado||operating|
|India||Indian Railways||67,368 km (41,861 mi)||operating|
|India||Delhi Metro||65 km (40 mi)||Phase-1 lines only; operating|
|India||Kolkata Metro||27.22 km (16.91 mi)||Line 1 only; operating|
|Iran||Zahedan to border with Pakistan||operating|
|Nepal||Nepal Railways||59 km (37 mi)||operating|
|Pakistan||Pakistan Railways||7,791 km (4,841 mi)||operating|
|Sri Lanka||Sri Lanka Railways||1,508 km (937 mi)||operating|
|United States||Bay Area Rapid Transit (BART) San Francisco Bay Area||109 mi (175 km)||operating|
|Canada||Grand Trunk Railway||converted to 4 ft 8 1⁄2 in (1,435 mm) standard gauge in 1873|
|Canada||St. Lawrence and Atlantic Railroad||converted to 4 ft 8 1⁄2 in (1,435 mm) standard gauge in 1873|
|Canada||Grand Trunk Railway of Canada||converted to 4 ft 8 1⁄2 in (1,435 mm) standard gauge|
|Canada||Intercolonial Railway of Canada||converted to 4 ft 8 1⁄2 in (1,435 mm) standard gauge in 1875|
|Paraguay||Paraguayan railway||From Asunción to Encarnación was originally laid in this gauge in the hope that the connecting line from Posadas to Buenos Aires would be built to the same gauge; that line was laid to standard gauge, and when the FCPCAL reached Encarnación in 1912 the whole line had to be re-gauged to standard gauge to allow through-working.|
|United Kingdom||Arbroath and Forfar Railway||see Scotch gauge, converted to standard gauge|
|United Kingdom||Dundee and Arbroath Railway||16 3⁄4 mi (27.0 km)||see Scotch gauge, converted to standard gauge|
|United States||Maine Central Railroad||converted to standard gauge in 1871|
|Wikimedia Commons has media related to 5 ft 6 in gauge railways.|