Commuters on the District Line might be concerned to learn two trains collided when the tracks moved.
The trains were travelling between East Putney and Southfields stations on May 1 when they caught each other, causing minor damage.
No-one was injured and there are no reports staff or customers were aware of the incident until the damage was discovered during a routine train check at the depot that evening.
The C Stock and new S Stock trains, which travel between Edgware Road and Wimbledon, were temporarily withdrawn from service on the branch while the incident was investigated.
Kevin Bootle, London Underground's (LU) district line manager, said: "It was found there had been a very slight movement of the tracks in this particular area which caused the trains to come into contact.
"London Underground engineers are now working to modify the track to ensure this incident is not repeated.
"As a precaution other areas of the branch were checked with no other issues found.
"The safety of our customers and staff is our top priority and we apologise for the limited disruption this may have caused."
Mick Cash, acting general manger of 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."
Normal operating hours have been restored.