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The last decade of the 19th century and the first of the 20th saw a spate of big cities - mostly in Europe and the USA - launch a new era of urban transit by opening underground rail systems. And we as travellers can be extremely grateful, as these wide-ranging Undergrounds make exploring many of the world's great metropolises marvelously convenient. Nowhere is that more true than in Spain's capital, whose Metro is the seventh longest globally, even though Madrid ranks just 50th in population. And this year the city has been pulling out quite a few stops for its Centennial - including myriad events (even a 5K race on streets running above key Metro lines) and a pop-up museum exhibition - since King Alfonso XIII officially inaugurated the Metro, changing the lives of madrileños for the better forever.