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

    Synonyms : metro, subway system, tube, underground
    Type Of : railroad, railroad line, railway, railway line, railway system
    Examples :
    • in Paris the subway system is called the `metro' and in London it is called the `tube' or the `underground'
  2. an underground tunnel or passage enabling pedestrians to cross a road or railway

    Synonyms : underpass
    Type Of : tunnel