5 ft 6 in (1,676 mm) is a broad track gauge, used in India, Pakistan, western Bangladesh, Sri Lanka, Argentina, Chile, and on BART (San Francisco Bay Area).
In Pakistan, all services currently operate on 1,676 mm (5 ft 6 in) broad gauge only, except for a 27.1 km (16.8 mi) line of Lahore metro.
[3] 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.
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.
[3] 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 120 miles (190 km) of double tracked routes.
The original engineers chose the wide gauge for its "great stability and smoother riding qualities"[4] and intended to make a state-of-the-art system for other municipalities to emulate.
Several Maine railroads connected to the Grand Trunk Railway shared its "Portland Gauge".
Only a few lines of the Ferrocarril del Sur (Southern Railroad Network) were 1,000 mm (3 ft 3+3⁄8 in) metre gauge or 1,435 mm (4 ft 8+1⁄2 in) standard gauge, 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.
For example, in recent years Chile and Argentina have bought second hand Spanish and Portuguese Iberian-gauge rolling stock.