What does the word subway mean?

Usage examples for subway

  1. " I shun the subway," he said. – From the Housetops by George Barr McCutcheon
  2. He gave up at last and invested half his small fund in a subway. – Victory by Lester del Rey
  3. " Just come with me," said the subway guard. – The Bobbsey Twins in a Great City by Laura Lee Hope