A man was injured after a youth threw his shoe and hit a man in the face, say British Transport Police.
The incident happened on board the Preston to Leeds train on April 30.
A spokesman for British Transport Police said: "An assault happened on board a train from Preston to Leeds.
"A group of youths were at Preston and one of them threw a shoe over his head which hit a man in the face. He sustained minor injuries."
No arrests have been made.
British Transport Police are investigating the incident and anybody with information can contact them on 0800 40 50 40 or text 61016.