Spain has some of the fastest growing transport networks in all of Europe and the entire world. The Madrid Metro, despite being in the 25th largest metropolitan area in the world, is the eighth largest subway system. Barcelona’s is also quite large. Several other cities either have Metro systems (i.e. Bilbao, Valencia, Sevilla) or they are under construction (Malaga, Vigo). Spain also has an extensive nationwide network. A number of high speed lines have been built in recent years at the same gauge as the rest of the European networks, which has facilitated international train traffic between Spain, France, and other points. Spain’s conventional trains run on a broad gauge, which made connecting the various networks difficult. In fact, four different gauges have been used on the various Spanish railway lines across the country.