Answer:
Madrid is not only the third largest capital in the EU (after London and Berlin) but it also has the third largest underground train network in europe, and the sixth largest underground system in the world.
Twelve dedicated lines criss cross the city covering any destination in Madrid you could think of. With economic prices this is the cheapest way to move around Madrid.
Every year a number of new stations and extensions to various lines will mean a faster and simpler cross city journeys. Opening times are from 0600 until 0200 daily.