More than 250 people had to be evacuated from Dalston Junction during rush hour after smoke from a nearby fire came billowing into the station.
A wooden shed in Ashwood Street went up in flames at about 8am, causing moke to enter the vents at the back of the station.
No injuries were reported but the closure has caused disruption for commuters, with cancellations expected until 10.30am.
London Overground has said lines are closed between Dalston Junction and Highbury and Islington, and National Rail say there will be no trains between Highbury and Islington and Shadwell until 10.30am.
The fire was brought under control by 9.30am.
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