Madrid Metro

The Madrid Metro is the large metro system serving the city of Madrid. It is one of the largest metro systems in the world, which is especially remarkable considering Madrid's population of approximately 3.5 million (Madrid city) to 6 million (metropolitan area). In fact, it is among the top 10 longest metros in the world, though Madrid is approximately the twenty fifth most populous metropolitan area in the world. It is also one of many fast growing systems in the world, rivaled, among many others, by the Seoul Metro or the Beijing Subway.

http://en.wikipedia.org/wiki/Madrid_Metro