Police are hunting for a man after a train passenger was headbutted and repeatedly punched for asking a fellow passenger to wear a face mask.
The victim was seriously injured in the assault on the Transport for London (TfL) service between Slough and Langley on 12 August.
Officers have released a CCTV image of a man they want to speak to.
It has been mandatory for people to wear face coverings on public transport in England since 15 June.
Some people are exempt, including those who cannot put on, wear or remove a face covering because of a physical or mental illness or impairment, or disability.
British Transport Police said a verbal altercation led to the assault.
"The man then squared up to the victim, before headbutting him to the floor and proceeding to punch him in the face numerous times," the force added.