Poirot moves on to interview another one of the first class passengers: Colonel Arbuthnot. Arbuthnot is a man of "true British brevity" (2.8.6). That is, he answers with yes's and no's.
Here's the rundown:
He is coming home on leave from England on a train, not the usual boat. Why? Reasons of his own, he says.
He stopped in Baghdad for three days. He claims not to have known Mary Debenham until the train.
Poirot asks his opinion of Mary Debenham, and says the crime was probably committed by a woman. (Yep, he's trying to push the Colonel's buttons.) Arbuthnot gets a little worked up and says the idea is absurd.
As for his actions last night, he tells the same story as MacQueen: the two of them were discussing Indian politics. Though the Colonel generally doesn't like Americans, he's made an exception for MacQueen.
Colonel Arbuthnot went to bed around 2 in the morning. MacQueen called the conductor as Arbuthnot left for his compartment.
He did get off with MacQueen at the Vincovi stop, but briefly, because of the blizzard.
Back in the compartment, he did indeed smoke a pipe. A pipe!
Did he see anyone pass by? No, he did not see anyone. He smelled a woman, though. He knows because it reminded him of the plight of women in Russia.
Arbuthnot has never been to America, but admits to knowing Colonel Toby Armstrong, Daisy Armstrong's father.
He says that if Ratchett was Cassetti, then he got what he deserved.
He also mentions something interesting: "trial by jury is a sound system" (2.8.90).
Arbuthnot mentions that he noticed that the door beyond his, No. 16, had a man peeping out of it when he got back to his compartment. The man looked suspicious.
Before he goes, Colonel Arbuthnot also mentions that Miss Debenham is a "pukka sahib." Poirot explains that this means her father and Colonel Arbuthnot's father went to same kind of school. But wait… aren't they not supposed to know each other?
Poirot and M. Bouc agree that, although they have a clue that points to Colonel Arbuthnot, he doesn't fit the crime. Each crime has a "signature" (2.8.121), Poirot tells us, and this one does not have Arbuthnot's.