Any Londoner can tell you that the Tube is overcrowded. But how many know the real reasons why? In the premiere of London Underground Revealed journey below London's pavements to look back over 125 years of crushed commuters and rush-hour rage and discover remarkable engineering history behind the biggest metro system in the Western world. With each day bringing over 4 million passengers into a vast network of tunnels under the city, the iconic transport system is creakingly close to capacity. Can an ambitious plot involving £16-billion to build 30 new stations drag it back from the brink?