Officers from British Transport Police (BTP) boarded a train at Wolverhampton on Sunday after receiving reports of the assault, quickly locating and arresting the suspect.
BTP has put out a message on social media that any incidents on public transport can be reported to BTP by texting 61016 and, in an emergency, always calling 999.
A spokesman for British Transport Police Black Country said: "Last night, officers at Wolverhampton responded to reports of a sexual assault on board a train.
"Officers arrived on scene within minutes of the report, locating and arresting the suspect."
