Thousands of train passengers could face severe delays until the end of the week - after "extensive" damage to overhead power cables.
The problems at Radlett in Hertfordshire mean services run by three train companies - Southeastern, East Midlands Trains and First Capital Connect (FCC) - are likely to be affected throughout today.
People could be delayed by up to an hour as there's major disruption expected on services between Luton and St Pancras International in London.
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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.