Underground railway systems in cities, or short for metropolitan areas. Refers to subway systems or urban regions containing a central city and surrounding areas.
Shortened from 'metropolitan', from Greek 'metropolis' meaning 'mother city'. First used for the London Underground, then adopted worldwide for urban rail systems.
The Paris Métro gave us the universal term for urban rail systems, though each city adds its own flavor - London's 'Tube', New York's 'Subway', Moscow's elegant 'Metro'. These systems often become the circulatory system of modern cities.
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