On Monday 3 August, between 6am and 6.30am, the victim, who was seated on the service as it travelled between Elephant and Castle and Balham stations, was approached by a man.
Without warning the man sat down and sexually assaulted her.
He then pulled drugs from his pocket and offered them to the victim before asking her to get off the train with him to take the drugs. He also stated he was in possession of a knife.
As the train arrived at Balham station, both the victim and the man disembarked the train separately. The victim left the station and walked down a nearby street and noticed the man was walking on the other side of the street.
She then walked back towards Balham station for safety and didn’t see the man again.
Officers believe the man in the CCTV image may have information which could help the investigation.
If you recognise him, or have any information, please contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 2000051915.