Poirot presents a list of things that need explaining. This is another list worth checking out in your book (3.2.1 and on).
It presents all the weird stuff in the case that needs explaining: the "H" on the handkerchief, the pipe cleaner, the scarlet kimono, the second conductor, the watch, the time of the murder, and the weird stab wounds.
First, the "H": it implicates Hubbard, Debenham (whose middle name is Hermione), and Hildegarde Schmidt.
Pipe cleaner: Constantine thinks it's faked, since no one could have left two clues on purpose like that.
The scarlet kimono? They have no idea.
The Wagon Lit conductor? The only people who could have worn it are the valet, Debenham, the Princess, and the Countess. But they all have alibis!
What about the watch? How did it get there? Was the murder committed earlier or later than 1:15 a.m.? The point is debated.
As for the wounds, they suggest that multiple people stabbed the body.
Poirot is not satisfied with the two murderers solution. Now, Poirot must sit back and think: "Which of them?" (3.2.50).