Commuters faced misery during evening rush hour after a points failure at Queenstown Road caused major delays on routes running from Waterloo to Clapham Junction.
South West Trains said it had made cancellations on routes running between Clapham Junction and Queenstown Road, after some London-bound lined were blocked.
It added that some trains needed to be terminated and restarted away from Waterloo station, at Clapham Junction and Richamond, while some services were cancelled altogether.
Disruption is expected until at least 8.30pm, it said.
#DisruptionSWT NEW - signalling issue (points failure) between Clapham Junction & Queenstown Rd, London bound line blocked (Windsor/Reading)— SWR Help (@SW_Help) May 15, 2017