Who is Mary Debenham, Colonel Arbuthnot from Murder on the Orient Express and what is their importance?

Mary Debenham and Colonel Arbuthnot present themselves as dispassionate British travelers, strangers to each other. Their true identities, however, are perhaps more complicated than those of the other passengers. Not only were they both affiliated with the Armstrong family (Miss Debenham as a secretary governess, and Arbuthnot as Col. Armstrong's best friend). They are also passionately in love and desperate to continue their relationship. During a private conversation overheard by Poirot, Miss Debenham urges Col. Arbuthnot to restrain his passion for her until "'it's all over ...'"