Transport cops received a report of the offence which they say happened at around 5.30pm on Monday, April 15 on board a train from Argyle Street to Bridgeton.
The man carried out the sex act under his trousers in front of a woman who was sitting opposite him.
Now the force has released a description of the man. He is described as white, around 5ft 9ins tall, with a slim build, a gaunt appearance and light brown hair
If you have any information regarding this incident, contact British Transport Police by texting 61016 or calling 0800 40 50 40 quoting reference 411 of 15 April.
Alternatively, you can call Crimestoppers on 0800 555 111.
