A 40 year-old woman was headbutted on a busy commuter train in Brixton on Thursday morning by a man carrying a work bench.
The woman needed treatment for a head injury at Brixton main line railway station in south London after the attack.
Rush-hour services run by the Southeastern train company were held up due to the incident, with ambulance, Metropolitan Police and British Transport Police (BTP) all attending.
BTP said its officers and Met officers had been called to Brixton station just before 9am "following a report that a woman had been assaulted on board a train".
BTP went on: "Medics from London Ambulance Service also attended and the woman, aged 40, was treated at the scene for a head injury."
BTP later said the woman had been headbutted but had been able to go home after treatment.
They added that officers were keen to identify a man described as black, aged between 35 and 45, around 5ft 9ins and of medium build.
He had short black hair with a goatee beard and was wearing a dark navy quilted jacket and dark trousers. He was also carrying a work bench.
Officers from BTP have launched an investigation and are urging anyone who witnessed the incident to contact BTP on 0800 405040.