People who "go in trains" cause late evening delays in the rush hour.
An official post posted on the National Rail website said, "An earlier incident on the London-Victoria-Balham railway line causes interference between these stations.
"The trains are now returning to normal, but can still be canceled or delayed while the service is being restored.
"Disruption is expected until 9pm."
Several reports from passengers on these trains have confirmed that the movement is still slow and in some cases non-existent.