Trains are delayed due to a signal fault.
One of the signals at Hurst Green, on the Kent and Sussex border, is stuck at red.
This is affecting trains from Oxted in Surrey to Uckfield and East Grinstead.
Southern Rail is advising commuters to allow ten minutes extra to their journey time.
A spokesman said: “One of the signals towards the direction of Uckfield and East Grinstead is 'stuck' at red. This means the driver has to make contact with the signaller to pass the signal at red. This process can take a bit of time so will cause delays to trains.
“This is at the junction were trains from Oxted travel towards either Uckfield or East Grinstead.
“There's no need to change your travel plans but you should allow up to ten minutes extra to your journey time.”
An update at 9.42am confirmed that the disruption at Hurst Green was now clear.
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