In the ending part, you will see that Poirot revealed the evidence that was done to throw off the police.

Then he showed the evidence that showed who masterminded the plot. All of them were in on the plot planned by the grandmother.

On the third set of evidence, the identity of the conductor is not really the correct one and that was revealed. So the rest were manipulated by one person in the third revelation.

Basically they were all in this together, but because Cassetti got what he basically deserved Poirot presents to the Police the evidence that incriminate someone other than the passengers(someone unknown). Furthermore you discover that the girl supposedly dead is not dead afterall (this is a major change from the book).Also Pierre Michel is one of the kidnappers who in the end loved the child and coudln't kill her.(another major change from the book)