Southeastern Rail said it was dealing with a "serious trespass incident" after the doors on a train were forced open.
A man is wanted for questioning over a series of "despicable" sexual assaults on buses, including on a 13-year-old girl, Scotland Yard said.
Thousands of passengers now face delays, with Southeastern trains warning "all services" would be affected.
Police have released a CCTV image of a 12-year-old girl who has been missing since Tuesday.
Suada Gashi, of Lewisham was last seen leaving Victoria coach station at 12.40pm on April 2, after getting off a coach from Birmingham.
Anyone with information should call the Missing Persons Unit on 020 8284 8455.
Thousands of demonstrators today marched in protest against plans to scrap Lewisham hospital's A&E department and downgrade its maternity ward.
A man has been arrested on suspicion of murder after the body of a woman in her 70s was found with multiple injuries.
The pensioner's body was found just after noon in Forest Hill in Lewisham, south London.
She was pronounced dead at the scene after London Ambulance Service was called.
Officers believe they know the woman's identity but have yet to inform next of kin or formally identify the body.
A post-mortem examination will take place in due course.
The man, in his 40s, is in custody at a south London police station.