Officers are appealing for witnesses after an indecent incident on board a Gourock to Glasgow Central service.
The incident happened around 7am, with police now appealing for information.
A man made prolonged eye contact with a 28-year-old woman before beginning to touch himself inappropriately, as the train left Paisley Gilmour station.
It is believed that the suspect left the train at the next station.
He is described as between 55 and 70 years of age, white and of skinny build and with thin white hair. He was wearing a flat cap, blue jeans, a black waterproof jacket and was carrying a black backpack.
Officers would like to speak to any passengers on board the service who witnessed the incident, particularly a female passenger in the same carriage and two other males.
They are urged to contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 172 of 17/10/2018. Alternatively they can call Crimestoppers on 0800 555 111.