London's Secret Underground: The Tunnel That Shouldn't Exist

For seventy-six years, a fully automated electric railway carried four million letters a day beneath the streets of London. It ran twenty-two hours a day, seventy feet below Oxford Street, under Selfridges, through Holborn, and on to Whitechapel. Almost nobody knew it existed. This is the story of the Mail Rail, from the Victorian traffic crisis that demanded it, through two world wars that tested it, to the closure that killed it and the explorers who brought it back from the dead.