Opened in 1863, London's subway came decades before underground mass transit projects in most other major cities.

While the early trains were steam powered, by 1890 electric locomotives were running on the tracks. It was the first electric underground railway, and greatly reduced pollution in the tubes.

During WWII, the stations served as makeshift bomb shelters. By the early 21st century, the system carried over a billion passengers a year on 250 miles of tracks.