noun (n)

• an electric railway operating below the surface of the ground (usually in a city)(noun.artifact)

  Example:
  In Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'.
  

  Synonym:
  metro, subway system, tube, underground
  

  source: wordnet30
• an underground tunnel or passage enabling pedestrians to cross a road or railway(noun.artifact)

  Synonym:
  underpass
  

  source: wordnet30
• An underground way or gallery; especially, a passage under a street, in which water mains, gas mains, telegraph wires, etc., are conducted.(noun)

  source: webster1913