People looking to travel by Southern Rail home tonight can expect delays.
A fault with the signalling system in the Streatham area is causing disruption between Clapham Junction and East Croydon/Mitcham Junction.
National Rail have advised that trains may be cancelled, delayed by up to 15 minutes or revised, and it is expected to last until at least 8pm.
"In this particular situation the fault has been confirmed as a 'track circuit failure'," National Rail posted on its website.
"This means the signalling system is unable to automatically verify if the next section of track past the affected signal is clear.
"Specialist engineers and signalling technicians from Network Rail are currently onsite and are attempting to rectify the fault so that trains can resume running at normal speed."
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