British Transport Police said the 15-year-old was attacked by a man and woman in their 40s.
The incident happened at Patterton station at about 23:30 on Saturday 3 October.
The teenager was punched from behind and stamped on as he lay on the ground, suffering facial injuries, including an injury to his eye.
He did not require hospital treatment.
His mobile phone was also stolen.
The boy had been with a group of friends on the 23:05 service between Glasgow Central and Neilston.
Det Con Ryan Tierney, of British Transport Police, said: "This would have been a busy service and I am aware there was an altercation between the group of younger boys and a group of older people shortly before the assault took place at Patterton.
"Anyone who was on the train and has information regarding those involved who remained on until it arrived at Neilston at 23:40 is urged to get in touch as a matter of urgency."
A teenager was assaulted and robbed as he got off a train in East Renfrewshire.