The incident reportedly happened on the evening of Monday, March 8.
A spokeswoman for the force said: "On March 8 between 8.30pm and 9pm, a man and a woman had an argument in a nearby bar before walking into Leeds station.
"It is reported the man then assaulted the woman, before boarding a train to York."
Anyone who witnessed what happened is asked to contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 740 of 08/03/19.
Those wishing to remain anonymous can also pass on information to independent crime charity Crimestoppers on 0800 40 50 40.