The Madrid Metro is the second largest underground system in Europe and the sixth largest in the world.

The first line of the Madrid metro opened on October 17, 1919, under the direction of the Compania de Metro Alfonso XIII, with 8 stations and a 3.5-km track.

This metro network now has 231 stations on 12 lines. It has 227 km of track and an additional 44 km of track is expected to be completed by the end of this year.

The Madrid Metro is one of the densest metro network in the world.

Image: Commuters walk into the subway at Plaza Colon in Madrid. | Photograph: Philippe Desmazes/AFP/Getty Images.