The Demon Train Arc is the seventh arc of Kimetsu no Yaiba.
After Enmu is assigned to kill Tanjirou Kamado in order to become an Upper moon and obtain more of Muzan's blood, Tanjirou, Zenitsu and Inosuke buy tickets for the Infinity Train in search for Kyoujourou Rengoku, the flame pillar. In Hope's that he has some info on the Dance of the fire God.
Rengoku informs them that he was sent on the train to kill a demon suspected to be inhabited within it, and that he knows nothing of the Dance of the fire God. He quickly senses the demon and kills it, the group, impressed by his skills, ask that he makes them his Tsugoko and Rengoku agrees.
As the trio, now with Rengoku sleeps, Enmu enlists the help of 5 civilians riding the train revealed to have severe Insomnia. One of which is the train conductor, who follows Enmu in order to dream of his now dead wife and child, Enmu promises to use his powers to give each of them heartwarming, sweet dreams in exchange for helping them carry 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, and the remaining 4 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 Tanjirou, Zenitsu and Nezuko as his minions. Kyoujourou dreams of his father and brother and his father's gradual fall into depression as well as how it affected the family, meanwhile Tanjirou dreams of being back together with his family.
Enmu's minions however, work on connecting themselves to their dreams. The rope used by Enmu's minions has a special magic that enables them to penetrate the dreams of others, the second stage of the plan is complete. Due to the nature of Enmu's powers, the dreams a person is shown exist in a limited space where outside there is an unconscious area where a person's "Spiritual core" lies. The unconscious area is an extension of the type of person someone is, practically a metaphorical representation of how they are and their nature as a person. Rengoku's unconscious area is portrayed to be incredibly hot, likely representing the burning passion of his personality, similar to the light of the sun.
Before Rengoku can have his "spiritual core" destroyed, thus rendering 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, rendering them unable to move within the dream. And Rengoku, due to being unable to kill Humans cannot move further, thus they are caught in a stalemate.
Meanwhile Tanjirou 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 demon slayer uniform that he is experiencing a dream.