Officers investigating the indecent incident on-board a train travelling between Leeds and Bradford have released a CCTV image of a man they want to speak to.
It happened at around 7pm on Saturday 23 October.
Leeds Festival road closures: Aberford village will be closed to festival traffic after being used as a 'rat run'
Murderer David Hill who bludgeoned his millionaire mother-in-law to death is released from prison on licence
Wakefield rugby club incident: Man arrested on suspicion of murder after victim dies following attack
Leeds drug farmer locked up after being handed 'lenient' sentence
Man dead after a paragliding accident near to Leeds
A British Transport Police spokeserson said a man approached the victim and her partner and told her she was pretty.
He then sat on the seat behind them and a short while later the victim saw that he was performing a sex act.
The spokesperson said: "Officers believe the man in the image may have information which could assist their investigation.
"If you recognise him, or have any information, please contact BTP by texting 61016 or calling 0800 40 50 40 quoting reference 2100079669.
"Alternatively, you can call Crimestoppers anonymously on 0800 555 111."
Support the YEP and become a subscriber today. Enjoy unlimited access to local news and the latest on Leeds United, With a digital subscription, you see fewer ads, enjoy faster load times, and get access to exclusive newsletters and content. Click here to subscribe.