Trains between Brighton and East Croydon are facing disruption this evening (Tuesday, August 27) due to a number of incidents today.
Thameslink said services between between these stations may be cancelled, delayed by up to 30 minutes or revised.
Disruption is expected until 8pm.
A number of routes and services are currently being affected.
Thameslink said a points failure at Preston Park has been rectified, so trains are once again able to run between Brighton and London, and between Hove and London.
But some services have already been significantly delayed, revised or cancelled due to this problem.
A signalling problem has also occurred just south of Purley, at Stoats Nest Junction.
Thameslink said this is causing significant delays for a number of northbound Southern, Thameslink and Gatwick Express services between Gatwick Airport and East Croydon, with a reduced service operating via these stations.
Southern and Thameslink have requested rail replacement buses from suppliers.