The big cat was exposed after it began behaving aggressively early today on a passenger train heading for the city of Yekaterinburg 1,400 kilometres east of Moscow, police said.
The lion cub's female owner "could not cope with the situation and it was necessary to urgently isolate the animal which was running wild," police said.
"The lion cub had first been kept in a cage but then the woman accompanying it for some reason let the animal out," a police spokesman told the Interfax news agency.
The woman faced questioning on "how she carried a 50-kilogramme, nine-month-old lion registered in travel documents as... A domestic cat," police said.
Police said they would also question rail staff who "without proper checks allowed cargo on board that was so dangerous for surrounding people.
