The Demon Train Arc is the seventh arc of Kimetsu no Yaiba.
The entire arc serves as the basis for the Kimetsu no Yaiba film.
After Enmu is assigned to kill Tanjiro Kamado in order to become an Upper Moon and obtain more of Muzan's blood, Tanjiro, Zenitsu and Inosuke buy tickets for the Infinity Train in search for Kyojuro Rengoku, the Flame Pillar. Tanjiro hopes he has some info on the Dance of the Fire God.
Kyojuro informs them that he was sent on a mission to kill a Demon suspected to be the cause for multiple disappearances on the train. He informs Tanjiro that unfortunately, he knows nothing of the Dance of the Fire God. He quickly senses a demon and kills it, the group, impressed by his skills, ask that he makes them his Tsuguko and Kyojuro accepts.
The trio and Kyojuro fall asleep and Enmu enlists the help of five civilians riding the train with severe insomnia. One of them is the train's conductor, who follows Enmu in order to dream of his late wife and child. Enmu promises to use his powers to give each of them heartwarming, sweet dreams in exchange for their help in carrying out his plan to kill the Demon Slayers. The group had already touched Enmu's blood by taking hold of their tickets, putting the first act of the plan into motion.
The conductor is put to sleep as a reward for his work, and the remaining four are sent to carry out the rest of the plan.
As they sleep, the group fall into vivid dreams as an effect of touching the tickets. Zenitsu dreams of being on a date with Nezuko, Inosuke dreams of being a "cave explorer" with Tanjiro, Zenitsu and Nezuko as his minions, and Kyojuro dreams of his brother and his father's gradual fall into depression as well as how it affected the family. Meanwhile, Tanjiro dreams of being back together with his family.
Enmu's minions work on connecting themselves to the group's dreams through a rope with a special spell that enables them to penetrate the dreams of others, completing the second stage of Enmu's plan.
Due to the nature of Enmu's powers, the dreams a person is shown exist in a limited space, outside being an unconscious area where a person's "spiritual core" lies. The unconscious area is described to be an extension of the type of person someone is, practically a metaphorical representation of how they are and their nature as a person.
Before Kyojuro can have his "spiritual core" destroyed (which would render him unable to move, think or feel, thus becoming easy prey), his body unconsciously moves and holds the person connected to his dream by the neck, preventing any damage to his core. Due to being unable to kill humans, however, Kyojuro cannot move further, thus putting both of them at a stalemate.
Meanwhile, Tanjiro carries out his dream while receiving more and more unconscious cues from his mind, such as seeing Nezuko's box or being told by himself in his Demon Slayer uniform that he is experiencing a dream.