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."

Deputy Education Minister Mduduzi Manana appeared in court last week to face the assault charge.
He is accused of hitting the woman during an argument at the venue.
On Saturday, a brief statement from President Jacob Zuma's office said he had accepted Mr Manana's resignation, and thanked him for "his contribution to the work of government".
Mr Manana has previously apologised for what he called a "shameful incident". He claimed he was provoked, but "should have exercised restraint".
His political party, the African National Congress, welcomed his resignation and also thanked him for his service.
When the allegations first emerged, the ANC said "such behaviour is unacceptable ... violence against women is the shame of our times".
The allegations caused widespread outrage in South Africa, where violence against women is not uncommon.
During his initial court hearing, he was brought into the courtroom through a separate entrance, and journalists were not permitted inside.
But Police Minister Fikile Mbalula insisted that Mr Manana would not receive any special treatment.
"The deputy minister will not be treated with special kid gloves but will face the full wrath of the law. The state fully supports the victim," he said.
He was released on bail of $375 (Â£290) pending further investigation.

Police said the firework was dropped through the doorway of a fish and chip shop by an "unknown male".
The incident happened at about 21:15 BST on Friday at the shop in Alliance Avenue.
The man was taken to hospital where he is described as being in a stable condition.
Condemning the incident, North Belfast SDLP assembly member Nichola Mallon said: "Fireworks aren't toys, they can cause serious injury and they can cause considerable damage to property and they need to be used responsibly.
"So I would send a very clear message to parents and to young people to realise the dangers of fireworks."
Sinn FÃ©in councillor JJ Magee said: "Hopefully the injured man will make a full and speedy recovery but it must have been a very traumatic experience."
Police have appealed for witnesses to contact them.
A teenager was assaulted and robbed as he got off a train in East Renfrewshire.
[NEXT_CONCEPT]
A South African minister has resigned amid accusations he assaulted a woman in a nightclub earlier this month.
[NEXT_CONCEPT]
A man has suffered burns after he was hit by what police have described as a "large firework rocket" in north Belfast.