A subplot where Ruth Kettering and Katherine Grey swap compartments is added, to introduce the possibility that Katherine is the murderer's real target.
Another subplot dealing with Ruth's Missing Mom is added to give Van Aldin a reason to go to Nice (in an attempt to prevent his daughter from meeting her mother).
For all of his faults, Derek Kettering does not add infidelity to his inveterate gambling and rude behavior.
Mirelle comes off as more sympathetic in the adaptation, since she's not Derek's mistress and is instead sent to seduce him by Van Aldin. After the plot blows up in her face, she lets Van Aldin have it by punching him in the face.
Joseph Aarons — whose character was already adapted out of the adaptation of Double Sin — is also adapted out here, and his skills at dealing with people in the dramatic profession aren't utilized at all.
Ada Mason's double identity as Kitty Kidd, the impersonator and actress, is left out, though her skills at impersonation are still present in the adaptation.
Bait-and-Switch: Lenox is found screaming and tells Poirot that she found Katherine dead. However, when Poirot boards the train to make sure, he bumps into Katherine who's safe and sound, indicating that it was someone else who got killed instead.
Better to Die than Be Killed: Major Knighton lets himself get run over by a train after being outed as the murderer.
Book Ends: The episode begins and ends with the Tamplins having fun at the French Riviera.
Brainless Beauty: Corky Evans is a male example. He's portrayed as a bit airheaded and at one point even mistakes a fake gemstone for the real thing.
Call-Forward: When Katherine tells Poirot she wants to travel further, she tells him she's heading to Vienna to board the Orient Express. Poirot reveals that he hasn't traveled on the train yet.
Damsel in Distress: Katherine Grey gets taken hostage by the murderer near the end of the adaptation. Fortunately, he relents and Katherine survives the ordeal.
Death by Adaptation: Major Knighton dies after getting run over by a train at the end of the climax.
Dies Differently in Adaptation: Ruth Kettering dies by getting her face smashed in rather than by strangulation, leaving her unrecognizable.
Everyone Is a Suspect: Even more so than the original. Many of the changes in the adaptation were done to give everyone — even those who weren't on the train in the original — a motive for the murder.
Five-Second Foreshadowing: One of the nuns caring for Dolores at the convent convinces her to turn over a pair of scissors. Right afterwards, Poirot finds several cuts on her arms, indicating she's harming herself.
Gory Discretion Shot: Ruth's mangled face is carefully left out of frame while Poirot inspects the crime scene. Similarly, we cut back to Poirot shielding Katherine after Major Knighton gets himself run over by a train.
Honey Trap: Defied. Van Aldin sends his paramour Mirelle to seduce Derek Kettering, in an attempt to gain evidence of infidelity that would be grounds for the latter's divorce from Ruth. Unfortunately for them, Derek has no interest in other women.
Hostage Situation: After Major Knighton is denounced as the murderer of Ruth Kettering, he tries to get away by taking Katherine Grey hostage and dares anyone not to take one more step closer or he will kill her. Poirot manages to talk him out of the situation, and finally, Knighton lets go of her before committing suicide by the Railroad Tracks of Doom.
Inheritance Murder: Discussed as a motive for the murder. Mirelle's motive is that as Van Aldin's lover, Ruth's existence was the major roadblock in her becoming Van Aldin's wife and inheriting his fortune. And since Katherine Grey is presented as a possible victim, her relatives, the Tamplins, would benefit from her untimely death given their Impoverished Patrician status.
Man Bites Man: Or rather "Woman Bites Woman". A mysterious intruder, later revealed to be the maid Ada Mason, approaches to kill Katherine Grey for having an affair with Major Richard Knighton and making her jealous; but as Katherine screams, Lenox Tamplin wrestles with Ada and bites her on the neck, making her retreat.
Meaningful Name: Ruth's mother is named Dolores, meaning "sorrowful". Not only is she separated from her daughter for almost her entire life, she's also clearly not in the best state mentally when we meet her.
Murder the Hypotenuse: Ada Mason attempts to murder Katherine Grey after her lover, Major Knighton, fell in love with Katherine.
Railroad Tracks of Doom: After a Hostage Situation with Katherine Grey fails, Major Knighton gets off the train and allows himself to get run over by another one on the tracks.
Red Herring: Since Ruth Kettering's corpse is found with a badly mangled head, the possibility that she faked her death to throw people off of her trail is discussed. Of course, this isn't the case and Ruth really is dead.
Servile Snarker: Major Knighton gets a dig in at his employer Rufus Van Aldin.
Van Aldin: I called you dull, Knighton, I apologize. I was just trying to make a point. Knighton: Oh, I've been called worse, sir.
Shout-Out: During one party scene at Ville Marguerite, the music that plays in the background is the 1936 song "Sing, Sing, Sing (With a Swing)" by Louis Prima.
Smash Cut: After Ada Mason says that she learned that Derek Kettering entered Ruth's apartment and Van Aldin asks for his arrest, we cut suddenly to Derek in jail as Poirot interviews him.
You Killed My Father: Discussed as Katherine Grey's possible motive for the murder. Since Van Aldin's ruthless takeover of Katherine's father's business led the latter to commit suicide, it's posited that Katherine would then commit Revenge by Proxy by killing Van Aldin's daughter Ruth.
