They should label certain documents before passing them off to some people in order to identify who left them on the train.
It's like teachers who pass out the tests. She makes 3 to 5 different sets of tests and passes them out to the class in the order of 1, 2, 3, 4, 5, 1, 2, 3, 4, 5, etc.
If anyone in class tries to cheat, they will only be getting answers from someone who has a different test. If a teacher checks the grades and finds some answers that seem out of sync, the student either tried to cheat or didn't study.