The 27-year-old male victim was punched three times in the face on a train travelling between Glasgow Central and East Kilbride on Saturday night.
The attack happened at around 10.25pm on the 10.14pm service as the train travelled in the Thorniebank area.
The victim suffered an injury to his nose and was released after treatment at hospital.
Constable Chris MacKellaich, the BTP office in charge of the enquiry, said: "The man responsible for the attack got off at Giffnock station.
"I am anxious to trace anyone who was on the train and can help identify him."
The suspect is between 40 and 50 years old and bald. He was wearing a white and blue striped shirt, and jeans.