Everyone gathers in the train car and Poirot gives two scenarios. Why two? You'll see. M. Bouc and Dr. Constantine will give their judgments concerning which is right. (They are the jury, in a sense.)
The first solution:
The murderer entered the train at either Belgrade or Vincovi dressed as a valet, murdered Ratchett and then left the train in plain clothes before it took off from the station.
But what of the watch? Poirot's stroke of genius: "Mr. Ratchett had omitted to put his watch back an hour as he should have done at Tzaribrod" (3.9.19). So the watch was ahead! Which means he died at 12:15.
M. Bouc doesn't know if he buys this: what about the voice in the compartment? Poirot explains: it was someone who came in and found Ratchett dead but was afraid to be implicated.
And Mrs. Hubbard? She was having a nightmare and got the time wrong.
And the maid? She did run into the man, but earlier, and had lied to give her lady an airtight alibi.
Dr. Constantine is mad and jumps up and says this does not hold water. What the heck? (The stab wounds, for example.)
Okay, okay, fine. Poirot gives the second solution:
Everyone on the train car was in on it. Where else would you find such a mix of classes and nationalities except in America? And that's what the Armstrong household was.
In other words, it was an elaborate scheme by all these people – all members of the Armstrong family or people affiliated with the Armstrongs – to bring justice to Daisy Armstrong.
The case included many red herrings: for example, MacQueen's suggestion that Ratchett couldn't speak French.
Then there were Mr. Hardman's methods. He only watched the hallway, which is not a good way to guard anyone, but it would keep an outsider from being implicated.
Also, the Colonel called Miss Debenham Mary, which means they were intimately acquainted.
Plus, Mrs. Hubbard's bolt on her door had not actually been covered by the sponge-bag. In odd number compartments, the bolt is above the handle.
The whole thing was a "comedy played for my benefit" (3.9.50), says Poirot.
They were all in on it, like a jury of twelve. And they all stabbed him, so as not to know who actually killed him.
The clues were dropped as red herrings to confuse matters and incriminate people who had airtight alibis.
The scarlet kimono? Probably the Countess's. That's another red herring.
The Countess was innocent, and her husband did the stabbing for her.
The conductor, Pierre Michel, was also in on it: his daughter was the nursemaid who threw herself out of the window.
Hardman was the one in love with the girl. (How sad!)
And Mrs. Hubbard? Why, she's none other than the famous actress Linda Arden herself.
At this, Mrs. Hubbard admits to it, and details the plot. They were, in fact, all in on it together. And they would have succeeded if it weren't for the accident – and Poirot!
Conclusion? The second solution is the one that is obviously correct. It matches up with the evidence. But has justice been served? M. Bouc and Dr. Constantine agree that it has, by choosing to present the first, false solution to the authorities.