It is alleged the attack happened after the woman was verbally abused by a group of young men.
Detectives with British Transport Police (BTP) are now appealing for the victim to contact them urgently after the incident was reported by a number of passengers on the train.
The incident happened on board the 5:42pm service from Glasgow Central to Paisley Canal last Wednesday.
Officers were called to Dumbreck station in Glasgow shortly before 6pm that day after receiving reports that three men were racially abusing a woman.
An “altercation” between the woman and the group then followed, resulting in the woman being head-butted.
Anyone who was on board the train is being urged to contact BTP.
The three men are described as being white and are thought to be aged between 16 and 18.
One was around 5ft 9’ and was wearing a grey hooded top and jogging bottoms. The second male was 6ft with short brown hair and had on a white and black tracksuit top.
The third male was 5ft 4’ and slim. He was wearing a dark hooded top and bottoms.
A BTP spokesman said: “Detectives are now appealing directly to her to get in touch as a matter of urgency.
“Officers are also hoping to identify any new witnesses who haven’t yet contacted police.”