A '''Subway''' is the North American term for a rapid transit system that runs underground for much or all of its route. Outside North America, they may be known as an underground, metro, or mass transit railway. Subways require large populations to run efficiently and run at a "grade separation" from other vehicles and run frequently. This means that they exist in the largest cities. | A '''Subway''' is the North American term for a rapid transit system that runs underground for much or all of its route. Outside North America, they may be known as an underground, metro, or mass transit railway. Subways require large populations to run efficiently and run at a "grade separation" from other vehicles and run frequently. This means that they exist in the largest cities. |