The victim was attacked at around 10.25pm on the 10.14pm service as the train travelled in the Thorniebank area.
He 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 alighted from the train at Giffnock station shortly after punching the victim in the face three times.
"I am anxious to trace anyone who was on the train and can provide information which can help identify him."
The suspect is described as being between 40 and 50 years old and he is bald. He was wearing a white and blue striped shirt, and jeans.
British Transport Police can be contacted on 0800 40 50 40 or by texting 61016. Information can also be given anonymously to the independent charity Crimestoppers on 0800 555 111.