The woman was slapped across the face by a man in the front carriage of a train from Glenrothes and Edinburgh Waverley on 22 October.
Police say the assault happened shortly after 9pm.
The assailant is described as white, bald and wearing a ‘Stone Island’-branded jacket, was believed to be with a larger group of men.
The woman got off the train at Dalmeny at 9.47pm, with witnesses describing her as ‘sobbing uncontrollably.’
Police say she has not reported the incident but is trying to trace her to find out what happened.
She was in her early or mid-twenties, with blonde hair and was wearing a bomber jacket and black leggings.
Investigating officer PC Liam Harper said: “We would like to speak to the woman who was hurt in this incident as we believe it left her extremely distressed.
“Violence of this nature is completely unacceptable and so I urge anyone who saw the incident, or knows what happened, to get in touch immediately.”
Anyone with information is asked to contact British Transport Police on 0800 40 50 40 or text 61016, quoting 157 of 24/11.