In the early 1900s, London's street traffic was so congested that mail delivery between central sorting offices and railway stations suffered severe delays. To solve this, the Post Office began constructing the Post Office Railway, later known as Mail Rail, in 1915. Opening in 1927, this 6.
5-mile narrow-gauge underground network operated entirely without drivers on board. The system achieved automation using a clever electromechanical design. Electrical power was supplied via a conductor rail divided into distinct insulated sections.
Dispatchers at trackside control frames controlled train movements by energizing or de-energizing specific rail segments. When approaching a station, trains automatically slowed down due to a steep upward gradient designed into the tunnel physics, and held at platforms using dead-man relay circuits. The driverless design allowed Mail Rail to run up to 22 hours a day, immune to London fog, snow, and surface gridlock, until operations were suspended in 2003.