While London boasts the world's oldest underground train network (opened in 1863) and Boston built the first subway in the United States in 1897, the New York City subway soon became the largest American system.

The world's first underground railway was the Metropolitan Railway in London, which opened on January 10, 1863. It used steam-powered locomotives to carry passengers between Paddington and Farringdon Street. While revolutionary, the steam made the tunnels notoriously smoky and uncomfortable. In the United States, the first subway was opened in Boston in 1897 (the Tremont Street Subway), followed by New York City's first line in 1904. The transition to the modern "electric" subway began in 1890 with London's City & South London Railway, which proved that electric traction was the only practical way to run deep-level "tube" trains without suffocating passengers. By 2026, these historic systems have grown into massive networks, but the original 1863 London brick tunnels are still a functional part of the modern Circle and District lines.