WARNING: The following contains major spoilers for Demon Slayer: Mugen Train.
After losing almost his entire family during a massive and traumatic demon attack, Tanjiro Kamado has spent the entirety of Demon Slayer thus far struggling and fighting for the chance to at least save his younger sister, Nezuko, by turning her back into a human.
Demon Slayer: Mugen Train takes his journey to the next level, forcing him to confront the events of his past and harden his resolve for the future, though he does gain help with the latter. With the assistance of the Flame Hashira, Kyojuro Rengoku, Tanjiro is left with parting words that will completely change him for the rest of the series.
Demon Slayer: Mugen Train takes place right where Season 1 left off. Tanjiro, Zenitsu and Inosuke are tasked with aiding Rengoku in stopping a string of mysterious disappearances occurring aboard the Mugen Train. As it turns out, the demon responsible is Enmu, Lower Rank One among the 12 Kizuki. He places the passengers under a deep sleep, allowing them to have blissful dreams that tempt them into staying in the dream world forever.
Tanjiro and Rengoku's first meeting -- before the events of Mugen Train -- takes place at the Ubuyashiki Estate, where Rengoku votes in favor of killing Nezuko. He's unable to accept Tanjiro's goal, even under his unique circumstances, due to his belief that the natural bloodlust of demons will endanger the lives of others. However, Rengoku remains neutral after Kagaya Ubuyashiki, the leader of the Demon Slayer Corps, accepts both Tanjiro and Nezuko.
It's only when they meet again on the Mugen Train that Tanjiro and Rengoku truly begin to open up and accept one another. After witnessing Rengoku's extraordinary swordsmanship abilities when taking down a demon, Tanjiro quickly comes to admire the Flame Hashira for his optimism and strength, even referring to him as an older brother. In turn, Rengoku offers to make Tanjiro his apprentice.
Near the beginning of Mugen Train, Tanjiro is forced to face his past through Enmu's dream. This solidifies the fact that he believes his life to be like living out a nightmare, constantly grappling with his anguish while living in fear of supernatural horrors like Muzan and his demons. This is why Tanjiro's dream shows his family alive and well, far away from experiencing life's cruelest tragedies.
When Tanjiro realizes what he's seeing is not real, he's overcome with immense guilt for not saving his family and the life he wishes he was still living. But instead of giving in to a false reality, Tanjiro is finally able to move past his trauma because he's already lost them and there's nothing he can do for them now. Instead, he can focus on himself as a demon slayer along with Nezuko, the one family member he still has a chance to save.
Despite this new resolve, Tanjiro is made acutely aware of his weaknesses as a novice demon slayer upon seeing the astonishing battle between Rengoku and Akaza, Upper Rank 3. Tanjiro and Inosuke can do nothing except watch, knowing their interference would only be a hindrance to Rengoku. As such, they watch helplessly as Rengoku is eventually struck down by Akaza, who flees into the forest before the sunrise can deteriorate his body.
After being deeply moved by Rengoku's determination, Tanjiro yells passionately of how Rengoku did not lose because he saved all 200 passengers aboard the train, while Akaza ran away. In his last moments, Rengoku offers Tanjiro, Zenitsu and Insouke words of encouragement. He entrusts them with final words to his father and younger brother and informs Tanjiro that his father might have the answers he seeks about the Hinokami Kagura dance. Rengoku believes in Tanjiro, encouraging him to keep moving toward the goal of becoming a Hashira -- and accepts Nezuko as part of the Demon Slayer Corps.
Though they were only on good terms for a matter of hours, Rengoku did much for Tanjiro, showing him there is still plenty of room to grow. While Tanjiro has struggled with his grief for a long time, Rengoku's valor prevailed to the very end. His short presence in Tanjiro's life, along with his final words, have resonated profoundly within Tanjiro and carved out a new path, strengthening his resolve to move forward. His life as a demon slayer can also contain great joy and success. After confronting his past, Tanjiro has now cemented his future and will always remember how Rengoku played a huge part in that.