London's Metropolitan Line was the world's first underground railway and is still used by 70 million passengers each year. Rob Bell discovers how the capital's overcrowding problem was the necessity that prompted this hitherto unthinkable transport innovation, which went into operation in January 1863, linking Paddington and a terminus at Farringdon. He fulfills his childhood dream of riding one of the original underground steam trains that carried passengers through tunnels and learns how unpleasant it must have been.