"It's crowded, uncomfortable and expensive -- but it defined London. And it's ours." (http://www.timeout.com/london/things-to-do/london-undergrounds-history-2)
London Tube circa 1900 courtesy https://en.wikipedia.org/wiki/History_of_the_London_Underground.
Charles Pearson first conceived the idea of "trains in drains" in 1845. Victorian London was growing at such a rapidk rate it needed a travel system to transport its citizens around the city. Excavation for the Tube or Underground began in 1860. Within three years, the Tube was ready for business, the world's first subway system. The train was a series of gas lit wooden cars pulled by a steam locomotive. The track spanned only 6 kilometres. Thirty eight thousand Londoners rode the line on the first day. By year's end, 9.5 million passengers had ridden the Tube.
The beauty of the Underground was that it was used by all classes. Author Henry Mayhew interviewed poor labourers who were relieved to be spared the 6 mile walk to and from work. Furthermore, they could live in two-room flats in the "suburbs" as opposed to one-room flats in the industrial centre. (https://en.wikipedia.org/wiki/London_Labour_and_the_London_Poor).
Prime Minister Gladstone opens the Metropolitan Railway circa 1863 courtesy http://www.gettyimages.co.uk/detail/news-photo/prime-minister-william-gladstone-opens-the-metropolitan-news-photo/463968281.
The Underground's foul, smoke filled atmosphere was an issue for some passengers John Fowler created a steamless locomotive, Fowler's Ghost, to reduce the amount of steam and smoke underground, an invention which did not take off. Smoking was banned on the carriages until an MP complained and each locomotive was assigned a smoking carriage. The London Tube owners allowed train drivers to grow beards in an effort to filter out the worst of the fumes.
In 1869, workers dug into the London clay under the river Thames to expand the Tube line from Great Tower Hill to Pickle Herring Stairs. This was followed by the Circle Line, Hammersmith & City, City & South Line, the Northern Line, Waterloo & City, Central, Bakerloo, Piccadilly & Charing Cross, Euston & Hampstead, all before 1907.
During the Second World War the London Underground housed an estimated 170,000 Londoners seeking refuge from Hitler's bombs. In fact, Time magazine claimed that "The Tube saved London during the Blitz." (http://time.com/3875448/london-underground-down-not-out-in-the-great-subway-in-world-war-ii/)
The London Tube saved the city during the Blitz circa 1940 courtesy
Today, the Undergvround tracks stretch 253 miles. At peak times, there are more than 538 trains circling the city. Four million passengers ride the Underground daily, the population of London at the turn of the last century. "[The London Tube] -- like Big Ben, , Tower Bridge and other landmarks -- has shaped non-Britons ideas of what London is." (http://time.com/3875448/london-underground-down-not-out-in-the-great-subway-in-world-war-ii/)