Iberian gauge (Spanish:ancho ibérico, trocha ibérica,Portuguese:bitola ibérica) is atrack gauge of1,668 mm (5 ft 5+21⁄32 in), most extensively used by the railways ofSpain andPortugal. Abroad gauge, it is the second-widest gauge in regular use anywhere in the world, with onlyIndian gauge railways,5 ft 6 in (1,676 mm), being wider (by8 mm (5⁄16 in)).

As finally established in 1955,^[1] the Iberian gauge is a compromise between the similar, but slightly different, gauges adopted as respective national standards in Spain and Portugal in the mid-19th century. The main railway networks of Spain were initially constructed to a1,672 mm (5 ft 5+13⁄16 in) gauge of sixCastilian feet. Those of Portugal were instead built to a1,435 mm (4 ft 8+1⁄2 in) and later railways to a1,664 mm (5 ft 5+1⁄2 in) gauge of fivePortuguese feet – close enough to allowinteroperability with Spanish railways.^[2]

Since the beginning of the 1990s newhigh-speed passenger lines in Spain have been built to the internationalstandard gauge of1,435 mm (4 ft 8+1⁄2 in), to allow these lines to link to the European high-speed network. Although the 22 km fromTardienta toHuesca (part of a branch from theMadrid toBarcelona high-speed line) has been reconstructed asmixed Iberic and standard gauge, in general the interface between the two gauges in Spain is dealt with by means of gauge-changing installations, which canadjust the gauge of appropriately designedwheelsets on the move.^[3][4]

Plans exist to convert more of the Iberian-gauge network in Spain and Portugal to standard gauge, an indication of which is the use, on several stretches of recently relaid broad-gauge track, of concrete sleepers pre-drilled with additional bolt holes allowing for repositioning of one rail to adjust the track to standard gauge (or todual gauge) or the narrowing of the gauge by moving both rails closer together maintaining the perfect alignment of theloading gauge.^[5][6]

TheIndian gauge (1,676 mm or5 ft 6 in) is closely similar, with only8 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 replacementwheelset may be required (for example, the Russian-Finnish train Allegro is built for a 1,522 mm track gauge, intermediate between the Russian 1,520 mm and the 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 somewhat narrower gauge, a strong wear of thewheels may occur if the wheelset is not replaced.