South West Trains are promising that commuter services will be "normal" in the morning, following hours of delays due to engineering work.
In a statement, the company said: "Safety is our absolute priority. We have no choice but to make sure these repairs are carried out before we can restore train services on these lines."
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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.