British Transport Police (BTP) officers are appealing for information following the incident, which happened around 1pm on 16 February.
The girl, who was with three other children, was at platform eight in the station when the attack took place.
Officers describe the suspect as being white with a heavy build and short dark hair.
He is said to have been wearing a black jacket, with a grey jumper, lilac shirt and grey trousers.
BTP have indicated they are keen to speak with anyone who witnessed the incident, and in particular, an individual who intervened.