A woman was verbally abused in a sexually explicit and offensive way on train travelling from Edinburgh Waverly to Bathgate.
The incident took place on board the train between 6.20am and 6.45am on 4 May.
The man – who was travelling with another man believed to be in his twenties – then left the train at Bathgate station.
He is described as being about 20 years old, 5ft 10ins, of a slim build with fair brown hair, an unshaven face and a West Lothian accent.
He was also wearing a navy blue hooded top, blue jeans and white trainers.
Anyone with information is urged to call 0800 40 50 40 or contact Crimestoppers anonymously on 0800 555 111.