The incident happened at around 10.15pm on Saturday, the 16th of September on the Balham bound service.
A girl was sat on the train with her friend when a man began talking to them.
He then sexually assaulted her and later exposed himself to her.
Anyone who recognises him is asked to contact British Transport Police by texting 61016 or by calling 0800 40 50 40 quoting reference number 744 of 16 September 2023. Alternatively you can call Crimestoppers anonymously on 0800 555 111.