Services are back up and running from Battersea Park station after four days of engineering work over the Easter weekend.
Engineers from Network Rail and its contractors worked through the Bank Holiday period to replace a complete junction at the station, with track and eight sets of points taken up and renewed.
The points allow trains to move from one track to another.
The work was completed on time despite the effects of Storm Katie, which blew debris onto the tracks on Monday morning.
Network Rail route managing director, Alasdair Coates, said: "I would like to thank passengers for their patience, and also our lineside neighbours – including the staff, dogs and cats of Battersea - who will have seen and heard our work going on.
"In two years we have replaced worn-out junctions at Keymer, Haywards Heath, Gatwick, Purley and now Battersea, which will improve passengers' comfort and also reliability.
"We run more than 6,000 trains very weekday in Kent and Sussex and it is this constant cycle of improvements that will keep our very busy railway running reliably well into the future."
A Southern spokesman said: "We'd like to thank our passengers for their patience and understanding while these critical improvements were made.
"Network Rail has replaced a part of the infrastructure which was in need of renewal and, like the new section at Purley, we fully expect this important junction to be more reliable, reducing the risk of failures and delays."
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