Police are now appealing for more information.
The incident happened at 6.35pm on Friday 29 November.
A man is reported to have indecently touched himself then exposed himself to a woman.
He left the train at Polmont station.
The suspect is described as white, 5ft 10, and aged in his late 20s. He had short dirty blonde hair and a beard that was either dirty blonde or ginger. He spoke with a Scottish accent.
He was wearing a green hooded top with white writing on the right sleeve. He was also wearing green cargo trousers and was carrying a grey holdall.
Anyone who knows the man is asked to contact BTP by texting 61016 or by calling 0800 40 50 40.
In both cases, quote reference number 591 of 29/11/19.
Alternatively, call Crimestoppers anonymously on 0800 555 111.