Two District Line Tubes collided between East Putney and Southfields stations on 1 May after a faulty track movement, it has been revealed.
The collision caused minor damages to carriages but there were no reported passenger injuries.
London Underground is investigating the incident and the two trains have been withdrawn from service temporarily.
London Underground (LU) District Line manager Kevin Bootle said: “The safety of our customers and staff is our top priority and we apologise for the limited disruption this may have caused.”
In a letter to staff he said: “LU engineers are now working to modify the track to ensure that this incident is not repeated.
“As a precaution other areas on the branch were checked with no other issues found.”
Mick Cash, acting general secretary for the RMT union, said: “We share the sense of relief that no passengers or staff were injured as a result.
“However, the incident once again shines the spotlight on the cuts to track inspections and maintenance at LU.”