The 19-year-old was on a service from Leeds to Huddersfield when a man took the adjacent seat.
PC Mark Hibbert, of British Transport Police, said: “As the train left Leeds, the man began to stroke her legs and thighs. She attempted to make him stop by moving her legs. The man persisted and continued to touch her throughout the journey till he got off the train at Huddersfield.
“The victim was understandably extremely shaken, and we would commend her for coming forward to report what must have been a very upsetting incident.”
BTP has released a CCTV image of a man they want to speak to over the incident, which happened at around 5.10pm on Monday, August 18.
“We take any form of unwanted sexual behaviour such as this extremely seriously, and we are committed to tackling these offences in whatever form they take,” PC Hibbert added.
Anyone who recognises the man should call BTP on 0800 405040 or Crimestoppers, anonymously, on 0800 555 111, quoting reference number PSUB/B9 of September 17.