Officers received a call just after 3 p.m. on Sept. 15 reporting an indecent act at the Downsview GO station, police say.
According to investigators, a man exposed himself to a 34-year-old woman on a train from the Union GO station to Downsview.
Police said the suspect boarded the train at around 2 p.m. from the downtown station and then exited the train at around 2:23 p.m. at the Downsview GO station.
Officers described him as a man in his mid-30s with short black hair and a medium build. He was seen wearing a white, short-sleeved V-neck T-shirt and tan pants and carrying a black backpack.
Anyone with information is asked to call police at 416-808-5200 or Crime Stoppers anonymously at 416-222-8477.