The incident took place shortly after 8pm.
PC Wayne Frost, investigating officer for the British Transport Police (BTP), said: "A woman was subjected to an alleged sexual assault as she disembarked from a train at Truro station at about 8.05pm on Saturday, 28 June.
"At this stage, I am appealing for any witnesses who were on the platform or who were alighting the train at the time of the incident, to come forward and make contact with British Transport Police.
"Incidents such as this are extremely rare on the rail network which is generally a safe environment on which to travel."
*A 25-year-old man was arrested on suspicion of sexual assault at the scene. He was later released on bail pending further enquiries until Wednesday, 16 July.
Anyone who witnessed the incident, or who has any information which could help the investigation, is asked to contact British Transport Police on Freefone 0800 40 50 40, or text 61016, quoting incident reference GSUB/B3 of 02/07/2014.
