M. Bouc and Dr. Constantine are totally confused. Poirot is puzzled, too, but he's more optimistic about being able to solve the case based on the evidence gathered from the passengers.
For example, MacQueen said Ratchett didn't know any foreign languages, and that's why he had a secretary, but the voice the conductor heard in the car was speaking French.
This tells us that at twenty minutes till one, some other person was in the compartment.
Also, what's up with the watch? Did someone stop the hands?
Finally, when could the intruder have gotten on the train? The only opportunity was when the train was stopped at Vincovi. So the killer must be one of the passengers.
Poirot presents a list of evidence that we have already heard with each passenger's motive, alibi, evidence against him or her, and any suspicious circumstances. Go back to your book and check it out – it sums everything up nicely (3.1.46-58).
Discouraged, M. Bouc says the list doesn't tell them much. With that, Poirot pulls out a list of questions that is a little more provocative.