Train services running through Clapham Junction were seriously disrupted on Thursday morning after a signalling problem was detected.
South Western Railway (SWR) said lines had reopened but services may stil be cancelled or delayed by up to 15 minutes, with disruption expected until 10am.
Engineers attended and all lines have now reopened, SWR said. The signalling fault was detected between Clapham Junction and London Waterloo.
Tickets are being accepted at no extra cost on TfL buses between London Waterloo and Wimbledon.
“We are very sorry for any delay that this may cause to your journey,” SWR said in a statement on Thursday.