The 23-year-old woman, from Hull, was on the train, which left Leeds just before 6.40pm on Tuesday, February, when a man sat next to her and tried to talk to her.
He then touched her thigh and leg repeatedly.
Detective Constable Ian Grice said: “This was unpleasant and scary experience for the victim. Nobody travelling on the railway should have to put up with this type of behaviour and we’re determined to find the man responsible.
“We believe the man captured on CCTV can help us with our enquiry. If you think you can name him, or if you have any information to help our investigation, we’d urge you to get in touch.”
Call 0800 405040 or text 61016, quoting reference 426 of March 9.