In most versions of Murder on the Orient Express, a director of the train train company (M. Bouc) gives up his 1st class accomodation to Hercule Poirot as he is alighting in Italy.
The wikipedia entry for this movie does not explicity cover this but when combined with the entry for the novel it seems clear.
Receiving a telegram from London about an impending case, Poirot must return home, with Bouc offering him a place onboard the fully booked Express.
On the second night of the journey, as he is only travelling to Italy, M. Bouc gives up his first class-compartment to Poirot, who is going to Calais. This gives Poirot the compartment next to Mr. Ratchett.
In the novel a "A.M.Harris" is listed but does not arrive.
“No. 7 berth—a second-class. The gentleman has not yet come, and it is four minutes to nine.”
“Who is it?”
“An Englishman,” the conductor consulted his list. “A M. Harris.”
“A name of good omen,” said Poirot. “I read my Dickens. M. Harris he will not arrive.”
“Put Monsieur’s luggage in No. 7,” said M. Bouc. “If this M. Harris arrives we will tell him that he is too late—that berths cannot be retained so long—we will arrange the matter one way or another. What do I care for a M. Harris?”
From the novel it seems that Harris was, in fact, non-existent. MacQueen attempts to protest when Poirot is assigned the compartment but the conductor is forced to confirm....
Poirot noticed the apology in his tone with some amusement. Doubtless the man had been promised a good tip if he could keep the compartment for the sole use of the other traveller. However, even the most munificent of tips lose their effect when a Director of the Company is on board and issues his orders.