A 17-year-old girl was on the platform towards Abbey Wood at Woolwich Arsenal station when she was sexually assaulted by a man at around 6.05am on July 19.
Both the man and the teenager then boarded a train around four minutes later.
Soon afterwards, the victim was raped in a carriage toilet.
The man left the station at Abbey Wood, while the girl continued alone to Slade Green station and later reported the incident to police.
On July 24, a 30-year-old man was arrested on suspicion of rape and taken into custody.
He was later released on bail pending further enquiries.
British Transport Police officers are investigating and are asking anyone who was at Woolwich Arsenal station and may have seen the pair on the platform, or the train, to contact police immediately.
You can text BTP on 61016, or call 0800 40 50 40, using the reference 288 of 22 July.
Alternatively, you can contact Crimestoppers on 0800 555 111 where information can be passed anonymously.
