The incident happened last Thursday after the 30-year-old victim boarded the 6.30pm Newcastle to Edinburgh Waverley service operated by Virgin East Coast.
She was unable to find a seat on the train and stood next to a group of men who she had been chatting to at Newcastle station before boarding.
During the journey British Transport Police (BTP) said one of the men sexually assaulted her.
In response the victim slapped him and the man retaliated by punching her on the jaw.
A BTP spokesman said: “Detectives are keen to speak with anyone who was on board this train and may have seen what happened.
“Any piece of information, no matter how small, could prove vital to the investigation.”
Anyone with information should contact BTP on 0800 40 50 40.