Opened in 1863, the railway, familiarly called the Tube, has 274 stations spread along 400km of lines.
The network runs deep under the British capital and is served by escalators that are among the longest in Europe.
Up to now, fires have created the biggest emergencies on the underground.
In 1984, a cigarette was believed to have sparked a blaze that swept through the Oxford Street station. No one was killed in that incident but smoking was banned on the network the following year.
In November 1987, however, a lit match ignited a wooden escalator panel and triggered another fire which killed 31 people.
A number of derailings have also occurred, highlighting the dilapidated state of the network.