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? Taking in the origins of the Underground, this one-off special also reveals how 19th century New York subway designs are inspiring the next generation of Tube trains, and explores the importance of early Parisian Metro makers to modern engineering methods. You'll also look at the continued technological advances stemming from the tragic 1987 Kings Cross fire, which will make the new Underground among the safest transport systems on the planet. Interviewees including Transport for London Network Services Director Nigel Holness and engineering expert Professor John Burland explain what the future holds for the capital's commuters as this historic subway system reinvents itself for a new era of 21st century travel.