Opened in 1927, the London Post Office Railway, known as Mail Rail, spanned 6. 5 miles beneath the city's streets to bypass severe surface traffic congestion. Operating 70 feet underground, this narrow-gauge electric network moved millions of letters daily between sorting offices and main rail terminals.
The system's engineering relied on clever electromechanical design rather than modern computers. Trains were completely uncrewed and governed by an electrified third-rail system divided into isolated power sections called blocks. Station switch cabins controlled momentum using gravity-assisted track design: tracks sloped upward as trains entered stations to naturally decelerate them, and sloped downward upon departure to assist electric motors in acceleration.
Reversible electric supply allowed station master operators to route trains onto side tracks or hold them at signals by cutting current to specific track blocks. Interlocking mechanical switches prevented collisions, while pneumatic brakes provided reliable stopping power. During World War II, the deep tunnels also served to protect invaluable national treasures, including the Rosetta Stone, from air raids.
Although operations ceased in 2003 due to cheaper road transport costs, Mail Rail stands as a pioneering marvel of early 20th-century industrial automation.