British Transport Police (BTP) is appealing for information after the man undressed before getting off a train at Church Stretton railway station.
Officers have released these CCTV images of a man they would like to speak to in connection with the incident, which took place around 6.40pm on September 11.
Investigating officer PC Kirk Smith said: "As the Cardiff to Manchester Piccadilly service was arriving into Church Stretton station, a man on board stripped off his clothes before casually walking off the train.
"The naked man, who had a rucksack on his back, was then seen to walk over the footbridge where he was met by another man. They both got into a white three-door hatchback car and drove away.
"The man may have thought his actions were nothing more than a joke, but in reality this was completely unacceptable and inappropriate behaviour which was clearly distressing for many of the passengers and rail staff present.
"Enquiries have been ongoing and I am today able to release CCTV images of a man I would like to identify and speak to, as I believe he may have key information in relation to this incident.
"We treat incidents of this nature very seriously and would encourage anyone who may know the man pictured or his current whereabouts to contact us."
Anyone with information should contact British Transport Police on 0800 40 50 40 or text 61016 quoting background reference WSUB/B8 of 10/11/2014.
Information can also be passed to the independent charity Crimestoppers on 0800 555 111.