ask a differrent
It is the french woman. Do it all and then you have to jump on buildings from guards.
you have to get all the clues and find the mystery passenger and then the train stops and you will know :) :)
In the back of the train
you find the key
The thief is a professional at picking locks. If you look around his cabin you can find lock pickers.
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.
you die
You go talk to everyone
you find the coal smudges on one of the luggage cases after the train stops unexpectedly
tell the train driver stop the train until you find her key
its behind the birdie poster on him wall.
you give him prune juice and you find a luggage key.