A woman has told of her shock at seeing a young woman performing a sex act on a man on a peak hour train heading to Armadale on Thursday afternoon.

Kerry described the incident to Radio 6PR as "flagrant and out there" saying the pair seemed to be part of a larger group which made her feel "quite intimidated."

Kerry noticed the lewd act around the Kelmscott line and said it continued until she got off the train in Armadale.

She had been travelling home from work on a train that arrived in Armadale at 5.37pm.

"The other ones [in the group] were all drinking, a few threats [were] thrown around to people, you know, if anyone sort of looked over, which was really hard not to do," Kerry said.