1. An underground train, especially common such large cities.
2. An underground way or gallery; especially, a passage under a street.
Or, in uk, underground; Rail service that runs underground. The first underground line in the world was in London. Opened 1863, it was essentially a roofed-in trench. The London Underground is still the longest, with over 250 mi/400 km of routes. Many major cities throughout the world have extensive systems; New York's subway system and Moscow's each handle millions of passengers a day.