5 ft 6 in gauge railway

1680 mm is a broad track gauge, used in India, Pakistan, western Bangladesh, Sri Lanka, Argentina, Chile, and on BART in the San Francisco Bay Area.

In North America, it is called Indian, Provincial, Portland, or Texas gauge. In Argentina and Chile, it is known as "trocha ancha" (Spanish for "broad gauge"). In the Indian subcontinent it is simply known as "broad gauge". It is the widest gauge in regular passenger use anywhere in the world.

India
In India, the initial freight railway lines were built using standard gauge. In the 1850s, the Great Indian Peninsula Railway adopted the gauge of 1676 mm for the first passenger railway in India between Bori Bunder and Thane. This was then adopted as the standard for the nationwide network.

Indian Railways today predominantly operates on 1676 mm broad gauge. 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 1676 mm broad gauge.

Bangladesh
Bangladesh Railways uses a mix of 1676 mm 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.

Nepal
In Nepal, all services currently operate on 1676 mm broad gauge only.

Pakistan
In Pakistan, all services currently operate on 1676 mm broad gauge only, except for a 27.1 km line of Lahore metro.

Sri Lanka
In Sri Lanka, all services currently operate on 1676 mm broad gauge only.

United Kingdom
The 1676 mm broad gauge was 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.

Spain and Portugal
The Iberian-gauge railways, that service much of Spain and Portugal, have a track gauge of 1668 mm (2 varas), just 8 mm different from 1676 mm. Used rolling stock from Iberia has been employed on broad-gauge lines in Argentina and Chile.

Canada
Canada became the first British colony, in the 1850s, to use broad gauge. It was known as the "Provincial gauge" in Canada.

The earliest railways in Canada, including the 1836 Champlain and St. Lawrence and 1847 Montreal and Lachine Railway however, were built to.

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.

United States
The Bay Area Rapid Transit system is the only operating railroad in the United States to use broad gauge, with 120 mi of double tracked routes. The original engineers chose the wide gauge for its "great stability and smoother riding qualities" and intended to make a state-of-the-art system for other municipalities to emulate. The use of broad gauge rails was one of many unconventional design elements included in its design which, in addition to its unusual gauge, also used flat-edge rail, rather than typical rail that angles slightly inward (although the shape of BART wheels and rail has been modified since then ). 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 broad gauge until 1872, and the Texas and New Orleans Railroad used  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 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. John A. Poor's chief engineer Alvin C. Morton compiled the following advantages of "Portland Gauge" for Maine railways in 1847:
 * Frost heaves (swelling of wet soil upon freezing) produce an uneven running surface causing an irregular rocking motion as trains moved past. A wider wheelbase offered a steadier ride with less wear on the machinery and roadbed.
 * Wider cars offered more room for passengers and cargo. Train length would be reduced for cars carrying the same amount of cargo. Shorter trains would lessen the effects of side winds, and permit more efficient application of power.
 * Wide gauge locomotives offered more room to place reciprocating machinery inside, rather than outside the driving wheels. Reciprocating machinery was a source of vibration before mechanical engineering encompassed a good understanding of dynamics; and keeping such vibration close to the center of mass reduced the angular momentum causing rocking.
 * Wider fireboxes and boilers allowed more powerful locomotives. The alternative of longer boilers held the disadvantage of poor firebox draft through the increased frictional resistance of longer boiler tubes.
 * More powerful locomotives carrying fewer, larger cars would have reduced manpower requirement for engine crews and shop personnel.
 * For locomotives of equal power, fuel consumption increased as gauge decreased, especially in colder outside temperatures.
 * More powerful wide gauge locomotives would be more capable for plowing snow; and thereby provide more reliable winter service.
 * Several gauges were in widespread use, and none had yet come into clear dominance.
 * Freight transfer was preferable to exchange of cars between railways because unowned cars were abused on foreign railways.
 * The Grand Trunk Railway system feeding the seaport of Portland, Maine offered little need for gauge transfer prior to loading on export shipping.
 * Potential advantages of freight transfer to the standard gauge railroad from Portland to Boston seemed insignificant as long as competitive rates were available for transport on steamships between the two ports.
 * The majority of Canadian freight anticipated to be carried over rail lines to Portland was heavy and bulky in comparison to its value, and must be transported cheaply in large quantities to maintain profitability for producers and transporters.

Argentina
The national railway network is predominantly on broad gauge.

Chile
Most links of broad gauge railways are in the center-south of the country. Only a few lines of the Ferrocarril del Sur (Southern Railroad Network) were or, the notable exceptions being one of the few active links: the Ramal Talca-Constitución branch and the Metro de Santiago. On the contrary, just a few branches of the FCN (Ferrocarril del Norte) were broad gauge, most notably the Mapocho-Puerto mainline between Santiago and Valparaiso, the Santiago–Valparaíso railway line. This link was directly connected to the southern railroad network using the Matucana tunnel that connected Mapocho and the Central Station in Santiago. The Transandine Railway that connected both Argentinean and Chilean broad gauge networks through the Uspallata pass in the Andes mountains was actually a narrow gauge link.

Similar gauges and compatibility
The Iberian gauge is closely similar to the Indian gauge, with only 8 mm 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, intermediate between Russian   and Finnish  ). 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.