Colonel Arbuthnot is a friend of Colonel Armstrong.

Colonel Arbuthnot, a lean, brown-skinned British officer of between forty and fifty, was the closest friend of Colonel John Armstrong. Their bond had been forged in the trenches: Arbuthnot himself stated that Armstrong had saved his life during the war, a debt of honor that would later shape his actions on the Orient Express. This friendship placed Arbuthnot at the heart of the Armstrong family tragedy, giving him a direct, personal stake in the fate of the man who had destroyed them.

Identity and Background

Arbuthnot was a career soldier with nearly thirty years' experience in India, a man of stiff upper lip and pronounced British prejudices. He disliked Americans as a rule, finding them sentimental and idealistic, and he held a low opinion of all foreigners. His manner was that of the pukka sahib—honourable, slightly stupid, and utterly upright. He had known Colonel Armstrong well, referring to him as "Toby Armstrong" and recalling that he was a "nice fellow" with a distinguished career who had won the Victoria Cross. This personal knowledge of the victim's family made Arbuthnot's later involvement in the conspiracy not merely ideological but deeply personal.

Role in the Conspiracy

Arbuthnot was present at the initial meeting in New York where the plan to execute Ratchett was conceived. Linda Arden, Mrs. Armstrong's mother and the actress who traveled as Mrs. Hubbard, recalled that Arbuthnot was there on "that awful day" and that he was "very keen on having twelve of us," insisting that the number of executioners should mirror a jury. He was uncomfortable with the stabbing method but agreed that it solved the practical difficulties of the plot. His role on the train was to provide an alibi for Hector MacQueen, Ratchett's secretary and another conspirator, by spending the crucial hours of the night talking with him in MacQueen's compartment. This alibi was mutually reinforcing: Arbuthnot vouched for MacQueen's presence, and MacQueen vouched for Arbuthnot's, creating a seemingly unbreakable chain of testimony between two men who appeared to be strangers.

Relationships and Deception

Arbuthnot's relationship with Mary Debenham was the most carefully concealed element of his presence on the train. Though they pretended to have met only days earlier on the journey from Baghdad, they were in fact well acquainted, and Arbuthnot was in love with her. Poirot overheard a fragment of their conversation at Konya—"Not now. Not now. When it's all over. When it's behind us"—which revealed their intimacy and their shared secret. Arbuthnot defended Miss Debenham fiercely when Poirot questioned her, calling her "a lady" and a "pukka sahib," and later threatened to break every bone in Poirot's body when the detective reduced her to tears. His protective fury was the only crack in his otherwise rigid composure, and it confirmed Poirot's suspicion that the two were far more than casual acquaintances.

Change and Ending

When Poirot propounded his two solutions, Arbuthnot remained silent, his face carved out of wood, rigid and impassive. He did not confess or attempt to justify his actions. It was Linda Arden who spoke for all of them, revealing that Arbuthnot had been present at the original planning and that his motive was rooted in the war bond between him and Colonel Armstrong. In the end, Arbuthnot accepted the first of Poirot's solutions—the one that blamed an outside assassin—as the official version to be offered to the Jugo-Slavian police. He thus escaped legal consequences, his role in the execution of Ratchett remaining a secret shared only among the twelve conspirators and the detective who chose to let them go.

Narrative Role and Significance

Arbuthnot functions in the novel as the embodiment of the British military code of honor—a man who believes in trial by jury and law and order, yet who participates in a premeditated lynching because his personal loyalty to a friend outweighs his abstract principles. His presence on the train is the emotional anchor of the conspiracy: he is not a wronged family member but a friend who acts out of love and duty. His stiff, unyielding exterior and his eventual, reluctant acceptance of the cover-up solution underscore the novel's central tension between justice and vengeance, law and loyalty.