Anna goes to find Sal and Jack, as one of them must be the second killer. Anna decides to confront Jack on her own with Reggie’s gun. Meanwhile, Seamus and Dante try to move Judd’s body. As they move through the observation car, the snow reminds Seamus of the time he got lost in a blizzard as a child. He was saved by his brother, Sean, and without Sean, he feels lost. Seamus’s hands are shaking badly, so he attempts to take a pill from his pillbox, but he hears Anna calling him. He follows her voice and finds Jack having a heart attack in his cabin. Seamus offers Jack one of his pills, which are muscle relaxants and could help stop his heart attack. Anna takes the pill, but before she gives it to Jack, she demands to know why he helped destroy her family. She asks him if he thinks about the men he helped kill. Jack admits to both his greed and his guilt, then Seamus gives him the pill. He breathes evenly again and tells Seamus and Anna that they should have let him die.

Seamus confronts Anna about her hesitation to give Jack the pill until after he answered her questions. Anna turns it around and asks him about the pills. Seamus admits to having had tremors for years. When he went to a doctor, he was diagnosed with a degenerative, terminal condition. When he told Anna he didn’t know what his future held after their revenge plot, he knew that he had no future. Anna promises to take Seamus to a better doctor, to fight for his health and his future, and Seamus kisses her. Anna tells him that she wishes she could reciprocate his romantic feelings, but she can’t. Seamus realizes his gun is gone. Seamus and Anna rush out of the cabin to search for it before they hear two gunshots.

Anna runs through the train. Reggie tells her to stay back, but when she sees Dante, she runs to him. Dante says Jack is still alive, and Anna checks on Sal. She finds Sal’s window shot out and Sal dead in her chair. Anna covers her with a sheet before accusing Dante of being the second killer and attempting to frame her for the murders. Dante denies it and begs Anna to believe him, but she demands that he be restrained in his cabin until they reach Chicago. Anna asks Seamus for a moment alone and lets the emotions wash over her.

Anna cries for five minutes exactly before composing herself. She doesn’t actually think Dante is the killer; she hopes that by accusing him she can protect him from the real killer. Anna slips into Sal’s room and listens through the wall as Jack argues with someone in his cabin. She plans to catch the killer in the act, just like Reggie suggested. As the argument intensifies, Anna bursts into Jack’s cabin. She finds Seamus smothering Jack with a pillow. Seamus apologizes and tells Anna that she wasn’t supposed to know.

Anna became convinced the second killer was Seamus. She knows he’s strong enough to commit the murders and has the motive, as when he and Anna first met, he told her that he wanted to be the one to kill those who conspired to kill his brother. Seamus promises not to attack Anna, who is hurt by his betrayal. He tells her that he only went along with Anna’s plan to turn the group over to the FBI because he loves her; he always wanted a more tangible revenge, so he killed one of them. He gave Jack a pill to prevent the heart attack because he wanted to kill Jack himself, though Anna saved Jack by interrupting. Seamus says it felt beautiful and freeing to take his own revenge. Anna assumes Seamus killed Judd, and she warns him that he’ll be arrested in Chicago. Seamus tells her that she doesn’t have to report him before returning her father’s pin to her. He tells Anna goodbye and gives her his gun before jumping off the train.

Anna screams in grief and frustration. She then realizes that Seamus only specified that he killed one person, so there is another killer on the train.

Everyone hears Anna scream. In Room C, Dante struggles against the ropes that bind him to his chair, untying himself. In Room B, Jack tries to calm himself down after Seamus’s attempt on his life. He feels so panicked that he wonders if he’s having a real heart attack after his two fake heart attacks. In Room A, Sal opens her eyes beneath the sheet.

The killer attempts to sneak up on Jack and kill the last conspirator. When the killer enters Jack’s cabin, they find Anna, who greets Reggie, revealing that Reggie is the other killer.

Reggie feels relieved to have been discovered. When he was 15, his father died. Reggie’s father was too old to enlist when the war started, but he decided to work as a railroad engineer and got a job driving a train of newly enlisted troops, the train that exploded and destroyed the Matheson family. Reggie was too young to enlist in the war, so he joined the FBI. He couldn’t believe his luck when the opportunity for revenge fell into his lap with his assignment to the Phoenix. He wasn’t sure what he was going to do at first, but when he stumbled upon the scene of Judd’s fake death, he knew Judd wasn’t dead. Reggie later snuck off to talk to Judd and hatched a plan to have Judd help him kill everyone and frame Anna. Reggie knew he could pass the blame onto Judd and make it appear that Judd intended to kill everyone and frame Anna. The plan went awry when Seamus discovered Judd alive and killed him.

Reggie asks Anna when she figured out that he was the killer. She replies that she noticed he changed his shirt after Herb’s death, because his shirt was soaked with blood from the murder and snow from climbing onto the roof of the train. She noticed the change in shirt when helping Reggie with his stab wound, which she then realized was self-inflicted, as the wound was on his left side. Judd was right-handed, so if he had snuck up behind Reggie and stabbed him, the wound would’ve been on Reggie’s right side.

Anna asks if Reggie plans to kill her if she doesn’t go along with his plan, and Reggie says he will shoot her if she tries to stop him. He tries to entice her to go along with his plan by promising this will be the ultimate revenge on Kenneth, who will be humiliated and imprisoned. Anna pulls out Reggie’s gun, which was in the pocket of his borrowed jacket. Reggie tells her the gun isn’t loaded, and Anna tells him that she’s not the only one who knows he’s the killer. Sal knocks Reggie out using a bottle of champagne.

Sal sets down the champagne bottle and wipes the fake bullet wound, made from lipstick and nail polish, off her forehead. Anna made a plan with Sal and Jack to out Seamus as the killer. She planned to fake Sal’s death and use Jack as bait to catch Seamus in the act. While Jack faked a heart attack, Anna distracted Seamus by asking Jack questions while Sal stole Seamus’s gun.

Anna thanks Sal for her help but makes sure to comment that they aren’t even. Dante finds them, and Anna fills him in about Reggie. Anna tells Sal, Jack, and Dante to lock themselves in their cabins and gives Dante Seamus’s gun to guard them. Anna goes to Reggie’s cabin and finds a box of bullets to reload Reggie’s gun. She finds an open window in Jack’s empty cabin where Reggie was previously unconscious. Reggie has climbed onto the roof.

Anna climbs out the window and follows Reggie onto the roof, though the snowstorm makes it difficult to navigate. She worries she’ll fall off the train, but she perseveres. She finds Reggie atop the train’s roof. She tells him not to come any closer and informs him that she’ll have to turn him in, pointing his loaded gun at him. He says he knows Anna won’t shoot him and tries to convince her to join him and give in to her violent revenge fantasies. Anna refuses, especially since it would mean killing Dante, and she corners Reggie on top of the observation car. She then shoots the glass roof, which shatters beneath them.

Anna and Reggie land in a sprawl in the observation car. They struggle for the gun, but Anna gets to it first. She keeps Reggie at gunpoint and mentally prepares herself to shoot him in the leg, but the train unexpectedly stops.

Reggie begins running out of the observation car. Anna chases him, but he hides in the club car to sneak up and attack her, taking the gun from her. She defends herself, throwing coffee cups to distract Reggie, before running toward the locomotive, hoping the engineer can help her. She pounds on the door until someone opens it, and she’s shocked to see Kenneth standing before her.

Anna is shocked to see Kenneth’s face, which frequently haunted her nightmares. She doesn’t know how to feel, but she refuses to flinch. Kenneth knows who she is and what she’s been up to; he fired Burt Chapman, the engineer, in Chicago and drove the train himself. He was tipped off by Anna bribing all his employees to not board the train, and he worked out her plan. The deaths of his co-conspirators works out well for him, as it buries more evidence of his crimes. Kenneth wanted what was rightfully his: Margaret and their son, Tommy. Anna realizes that she kept seeing Dante on the train and mistaking him for Tommy because they’re half-brothers. Anna is even angrier at Kenneth for daring to mourn Tommy after Kenneth was the one who killed him. Kenneth is angry that he was never allowed to acknowledge his son, as Arthur pretended Tommy was his. Kenneth also confesses to paying someone to murder Arthur in prison. If Arthur lived, he could have cleared his name, perhaps preventing Margaret’s death by suicide.

Anna seethes with rage as Reggie, who served as a witness to Kenneth’s confession, tries to goad her into shooting Kenneth. Anna demands that Kenneth admit he destroyed her family, and he does. Kenneth is afraid as Anna points the gun at him, begging her not to shoot. Anna is tired of holding all the rage and pain inside her. She wonders if killing Kenneth would put her life back together. She remembers Seamus’s insistence that she knows what to do, how to end this. She pulls the trigger.

Two dozen FBI agents swarm the arrival platform in Chicago as the Phoenix arrives. In Car 13, they find the bodies of Edith, Herb, and Judd in their rooms. In Car 12, they find Sal and Jack being held at gunpoint by Dante. They find Reggie in the baggage car with a gunshot wound to the leg, courtesy of Anna. They then find Anna and Kenneth at the front of the train, as Anna holds Kenneth at gunpoint. Anna tells the agents that Kenneth is responsible for the deaths of over 30 people, and they agree, as the evidence sent to the Chicago office is overwhelming. The agents tell Anna that they’re sorry for her loss, which is the first time anyone has said that to her besides Seamus.

The officers arrest Jack, Sal, and Kenneth. Sal nearly apologizes, but she and Anna have an unspoken moment. Jack cries as he’s arrested, and Kenneth argues that the evidence isn’t legitimate. After everyone is hauled away, Dante approaches Anna and promises he didn’t know his father was on the train. Anna believes him. The two decide to be friends, and Anna warns Dante about how cruel the press and public will be to him and his mother after the news about Kenneth comes out. Dante asks Anna if he can write to her about Tommy, their shared brother, and Anna agrees. She also tells Dante that Tommy would have loved him.

Reggie tells Anna that he confessed to killing Edith, Herb, and Judd, hiding the truth of Seamus’s involvement. Anna thanks him, and Reggie tells her to try to forgive Seamus for what he did. Reggie asks Anna why she wanted them all to live, but Anna doesn’t know. Reggie leaves, and his boss, Ed Vesper, walks up to Anna. He tells Anna that Reggie spoke highly of her skills and asks what her future holds. Anna is still uncertain about her future, just as she discussed it with Seamus, whom she secretly hopes survived the fall. Ed offers Anna a job as an FBI agent. Anna accepts, and they plan to fly back to Philadelphia. Anna is done with trains.