A man trapped a woman in a toilet on an early morning Brighton to London train.
British Transport Police (BTP) detectives want a man who may have seen what the force described as “a disturbing incident” to come forward.
The man made indecent remarks and grabbed her arm when she tried to escape.
The incident happened on Saturday (29 November) aboard the 3.50am Brighton to Victoria train as it travelled towards Haywards Heath.
Detective Constable Vanessa Cottington said: “Another man opened the door during the incident.
“We need him to come forward so we can establish exactly what happened.
“If the man is reading this, I would ask him to come forward by contacting BTP on 0800 405040 or text 61016.
“Information can also be passed anonymously to the independent charity Crimestoppers on 0800 555111.”
A 33-year-old man from London has been arrested on suspicion of false imprisonment in relation to this incident.
He has been bailed until Monday 26 January.