Underground railway system used to transport passengers within urban and suburban areas. The first subway line, 3.75 mi (6 km) long, opened in London in 1863 and carried 912 million passengers in its first year. The first electrified subway opened in 1890 in London (where it is called the underground or tube). Subways opened in Budapest in 1896 (the first on the European continent), Boston in 1897, Paris in 1900 (where it is called the métro), Berlin in 1902, New York in 1904, and later in Madrid (1919), Tokyo (1927), and Moscow (1935). Improvements in systems built from the 1970s on (including San Francisco, Washington, D.C., and Los Angeles) include computer technology to run subway trains by remote control, and refinements in track and car construction for faster, smoother, and quieter rides.

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• The Underground Railroad, a network of clandestine routes by which African slaves in the 19th century United States attempted to escape to free states, or as far north as Canada, with the aid of abolitionists.
• Rapid transit, urban railways that sometimes use tunnels.