British Transport Police (BTP) officers are appealing for witnesses after reports that a woman in her 20s was groped on a Glasgow Central to Lanark train at 8.51pm on Thursday July 11.
The suspect, described as a tall, white, bald man of heavy build and with tattoos on his hand, also made inappropriate sexual comments to the woman, according to police.
The man was wearing a purple or pink hooded top, jeans and green trainers, and was carrying a white drawstring bag.
Officers are appealing for anyone who saw the incident, or who may know the suspect, to speak to them.
Witnesses are asked to contact the force by texting 61016 or by calling 0800 405040 and quoting reference 100 of 12/07/19.