In the novel The Mystery of the Blue Train, Rufus Van Aldin is an American millionaire. He is the father of Ruth Kettering. He is a friend of Marie Van Schuyler.
His London solicitors are Messrs Galbraith, Galbraith, Cuthbertson, & Galbraith.
Mr Van Aldin is described as being a "big, broad-shouldered man". As a young man, he had red hair, and had been called Carrots and Ginger. He describes himself as "pretty tough", and attributes his good health to living a simple life, eating plain fare and not too much of it.
He seldom expresses blame or praise, and gives his employees a fair trial, promptly dismissing those who are inefficient. His selections of people are unconventional. For example, he had hired Major Knighton as his secretary after meeting him casually at a Swiss resort.
At the beginning of the novel, Mr Van Aldin buys Heart of Fire, along with other rubies, as a surprise for Ruth. He nonchalantly tells Major Knighton that he paid four or five hundred thousand dollars for them, and he is able to use his revolver to fend off two apaches who attack him.
Mr Van Aldin receives a letter from Ruth, and when he sees it, his face is "suddenly trransformed". The harsh contours soften, and the hard line of his mouth relaxes. He tells Major Knighton that Ruth is his only child, and no one knows quite what she means to him. He is of the opinion that he can buy a good portion of the earth for Ruth, but because she is not happy in her home, it will not do any good.
Mr Van Aldin advises Ruth to divorce Derek Kettering, saying that Derek is no good. He says that he blames himself for letting Ruth marry Derek, but he had crossed her once when he had made her break off her relationship with the Comte de la Roche, and was too soft-hearted to do it again.
Mr Van Aldin gives Ruth the rubies, and advises her to leave them at the bank when she travels to the Riviera. He jokes that they do not want her robbed and murdered for the sake of "Heart of Fire".
Mr Van Aldin employs Mr Goby to get information on Derek Kettering. He later tells Derek that Ruth proposes to divorce him, and warns him not to defend the case. After finding out that Ruth has been seeing the Comte de la Roche again, Mr Van Aldin sends Knighton to make Derek an offer of a hundred thousand pounds if he will not defend the divorce case.
Mr Van Aldin goes to Victoria to see Ruth off, putting off a conference because she is more to him than "any number of darned conferences".
After Ruth's death, Mr Van Aldin receives a telegram informing him about it. He travels to Nice with his valet, Archer, and Major Knighton, stopping by in Paris to pick up Ada Mason. In Nice, he meets Poirot, and asks for his help in solving the case, saying that "one big man can ask a favour from another big man".
Mr Van Aldin asks Katherine Grey to call on him, because he wants to hear what she can tell him about Ruth. He does not show any outward sign of grief, but Katherine is of the opinion that this makes his grief seem even more real. Mr Van Aldin thanks Katherine for easing Ruth's mind in the last hours of her life. He also asks her if Ruth might have changed her mind about meeting the Comte de la Roche after their conversation.
Mr Van Aldin disapproves of Poirot's idea to go to the tennis, saying that in the States, business comes before pleasure. He briefly holds the opinion that Poirot is past his job, but reverts to his original good opinion after Poirot points out M. Papopolous, who has come to Nice. When Poirot shows him what he believes to be the rubies which were stolen from Ruth, he wrings Poirot's hand so heartily that Poirot winces.
Mirelle later calls at the hotel, saying thatbshe must see Mr Van Aldin immediately. However, Mr Van Aldin is furious, and refuses to see her, sending Major Knighton down to tell her so.
Mr Van Aldin is very distressed after the arrest of Derek Kettering, and returns to England. When Major Knighton asks if he would have wanted Derek to go scot free, he says that he would have preferred to take the law into his own hands.
Mr Van Aldin and Major Knighton go with Poirot to France, travelling on the Blue Train. Poirot suddenly says that they must get off the train in Paris, but they are held up because their tickets are with the train conductor. Poirot then decides that they must get back on the train and continue their journey quietly. Mr Van Aldin remarks privately to Major Knighton that Poirot has lost his grip, and that any man who "scuttles round like a frightened rabbit is no earthly damned good".
Mr Van Aldin is present when Poirot reveals that Major Knighton was the one who murdered Ruth. Later, whenhe and Poirot are at his private suite, he asks Poirot for explanations. After Poirot explains how he solved the case, Mr Van Aldin says that he will send Poirot a cheque, but that no cheque can express his feelings about what Poirot has done for him.