London Underground says that up to 5,000 people are at work under London's streets every night to work on maintaining the tube network. These essential works mean that the tube cannot run 24 hours a day.
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Paramedics were on standby outside the police cordon as armed officers stood by the property.
A team of researchers has taken samples from the ten busiest London Underground lines to work out which are the dirtiest.
The action comes on top of an emergency timetable which has seen hundreds of trains a day being axed.