The female, who is believed to be a teenager, was approached by a man and touched inappropriately on a platform at Newton station about 9.45pm on Saturday, July 28.
The incident was reported to British Transport Police by a passenger waiting at the platform and other passengers later got in touch. However, BTP said the victim is yet to report the incident.
The suspect is described as white, of slim build and with short dark hair. He had stubble and was wearing a black hooded top with a large white Adidas logo, black ripped jeans and black shoes.
A BTP spokesman said: “Other passengers have since contacted BTP to report this matter. However, the victim - who is believed to be in her teens - is yet to report the incident to police.
“Officers are appealing for any other witnesses to get in touch, likewise if you are the victim in this case then please get in touch as soon as possible.
“Anyone with information can contact BTP by sending a text to 61016 or by calling 0800 40 50 40 quoting reference 1800055102.
Alternatively call Crimestoppers in confidence on 0800 555 111.”