In the early 1920s, London's surface traffic was notoriously congested, slowing down letter and parcel deliveries between sorting offices and major railway hubs. To bypass the gridlock, the Post Office undertook a daring engineering project: building an 11. 5-kilometer narrow-gauge underground electric railway, officially known as the Post Office Railway and later dubbed Mail Rail.
When it opened in 1927, it featured a revolutionary technological feat: the trains had no drivers onboard. Operating 21 meters below the surface, the Mail Rail system relied on an ingenious electromechanical automation scheme. Trains ran on a 610 mm (2-foot) narrow gauge track powered by a third-rail electrical system.
Control was entirely handled from dispatched switchboards at individual stations. Electrical tracks were divided into isolated power blocks. Station signalmen controlled the track switches and energized specific rail segments to move, slow down, or stop trains automatically.
At the approach to stations, tracks sloped upward to naturally slow down incoming driverless trains using gravity, while departure tracks sloped downward to assist acceleration and conserve electricity. At its peak, the driverless network operated 22 hours a day, hauling four million letters daily across eight stations ranging from Paddington to Whitechapel. During World War II, the deep tunnels even served as a secure hiding place for precious national art treasures, including the Rosetta Stone.
The system operated reliably for 76 years until its closure in 2003, serving as a pioneer of industrial automation.