"Metro" (short for metropolitan railway) is the generic term for urban rapid transit systems—electric trains operating on grade-separated rights-of-way (subways, elevated lines, or surface corridors with no level crossings).
London Underground (1863): World's first, "The Tube," 272 stations, 402 km.
New York City Subway (1904): Largest by stations (472), 399 km, 24/7 operation.
Paris Métro (1900): Dense core network (308 stations, 225 km), Art Nouveau entrances.
Tokyo Metro/Toei (1927): World's busiest (8.7M daily pre-COVID), legendary punctuality.
Over 180 cities worldwide have metro systems. China leads new construction: Shanghai (world's longest at 803 km), Beijing, Guangzhou, Chengdu, Shenzhen all opened post-1990. Emerging networks in India, Southeast Asia, Middle East, Latin America.
"Subway" (US), "Underground"/"Tube" (UK), "U-Bahn"/"S-Bahn" (Germany), "Metro" (most Romance languages, Russia, China), "MRT" (Singapore, Taiwan), "Subte" (Buenos Aires). All denote the same technological category.