Mystery Train island is came out on the 9th of September. Only members were aloud to play the whole island. Nonmembers had a demo. Mystery Train Island is coming out on the 7th of October for everyone.
As most people who play the game know, the thief is the reporter for the newspaper Le Monde. As for what good that does you, here is the link to the site for Poptropica, which gives all kinds of helpful hints and clues.
After you collect all the clues, you arrive at the Chicago World's Fair just as you discover that the thief is Mademoiselle Moreau, the French reporter. Follow her through the Fair and up the Ferris Wheel to recover the stolen transformer. (see the related full solution question)
You click on the map (it is in the right hand side corner at the top), and at the bottom left hand side there is a blue circle that says Restart Island. Click on it and you will restart the island. (Notice that it is not the map of all the islands but just the island map of Mystery Train.)