US Cities with Underground Subway Systems

Ten US cities operate heavy-rail subway/metro systems with significant underground segments:

1. New York City (MTA Subway — 472 stations, largest)
2. Chicago (CTA 'L' — Red/Blue Lines subway)
3. Philadelphia (SEPTA Broad Street & Market-Frankford Lines)
4. Boston (MBTA — Red, Orange, Blue Lines)
5. San Francisco (Muni Metro subway + BART)
6. Washington, D.C. (WMATA Metrorail — extensive subway)
7. Los Angeles (Metro Rail — B/D Lines subway; expanding)
8. Atlanta (MARTA — heavy rail subway)
9. Baltimore (Metro SubwayLink — single line)
10. Cleveland (RTA Red Line — partially subway)

Notable Exclusions

• Light rail/streetcars (Portland, Seattle, Denver, Minneapolis, Dallas, Houston, Phoenix, San Diego, etc.) — not heavy rail subway
• PATH (NYC–NJ) — rapid transit but interstate
• PATCO (Philly–NJ) — similar

NYC has the only 24/7 system. Others typically close 12 AM–5 AM.