1. underground railway systems for public transportation in cities
2. underground passages or tunnels for pedestrians

Examples

• The city’s subways run 24 hours a day.
• Many major cities have extensive subway systems.
• She takes the subway to work every morning.
• The subway station was crowded during rush hour.
• Pedestrians use subways to cross busy intersections safely.
• The subway under the highway connects both sides of town.
• New York’s subways are among the busiest in the world.