The victim, a 20-year-old woman from Edinburgh, had first been approached by a man who attempted to engage her in conversation whilst she was waiting to catch a train at Harrogate station on Friday 16 September 2016.
He then closely followed her onto the platform and onto the 1.06pm service from Harrogate to York, where he sat next to her.
The man then placed his head on her shoulder before inappropriately touching her legs and thighs.
Investigating Officer PC Robert McDonald said:
This was an extremely upsetting incident for the victim, who was so distressed she has only just been able to provide us with a statement.
I am keen to speak with this man in the CCTV image, who left the train at York as I believe he has essential information that will assist our enquiries.
We simply will not tolerate unwanted sexual behaviour of this nature on the rail network. If you are ever a victim of sexual assault, please report it to us so we can trace and stop these perpetrators.
Anyone with any information can contact BTP by sending a text to 61016 or by calling 0800 40 50 40 quoting reference 294 of 4/11. Alternatively, information can be passed anonymously to independent charity Crimestoppers on 0800 555 111.