Flooding, track and signal faults have seen trains travelling through Sutton train station being cancelled or delayed by up to 45 minutes.
A points failure between Balham and Victoria stations and a track defect near Purley this morning forced trains to use slower routes than normal.
Torrential rain bring flooding to Sussex
Two cases flooding near Haywards Heath and Lewes in Sussex saw trains being re-routed, which added to the delays.
But a spokeswoman from Southern Railway said she expected regular service to resume after 5pm.
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