A 17-year-old girl was sexually assaulted twice, by two different men, as she made her way home by train from a popular music festival.
As she boarded the train, a man in a novelty shark hat approached her and asked if she needed help with her luggage.
When on the train, he sat next to her and then asked if she wanted a kiss.
After she said no, the man started tickling her and attempted to take her phone. The girl then moved to a different seat.
She was then approached by another man, who she told what had just happened.
The man, who gave his name as Lance, sexually assaulted her by kissing her and touching her and himself inappropriately.
The incidents happened between around 8.45pm and 9pm on August 12.
British Transport Police investigating officer Detective Constable Matt Grieve said: "This was a deeply upsetting and shocking incident for this young woman and we are urgently seeking witnesses.
"If you were on board this train and witnessed what happened, or saw two men acting suspiciously, then I would urge you to get in touch.
"Your information could prove vital in our enquiries to understand what happened, and who is responsible."
The first offender is white with pale skin and ginger hair. He was wearing a white shirt with a palm tree on it, blue shorts and a novelty shark hat.
The second offender is also white, with mousy hair and pronounced sideburns. He had on a white polo shirt and camouflage shorts.