The 58-year-old victim boarded the 8.18pm service from Haymarket to Inverkeithing at the same time as another man on Monday night and an argument broke out between the pair. The victim received several punches to the head and the suspect ended up being pulled off him by another passenger.
Both men got off the train at Inverkeithing just after 8.30pm and the victim was taken to the Victoria Hospital in Kirkcaldy for treatment to his injuries, including a broken nose.
Constable Thomas Ashman, a BTP officer said it was an "unwarranted and violent" attack. The suspect is believed to live in Inverkeithing as he regularly commutes from there.