Infinity Train Wiki
Infinity Train Wiki

Book Four - Duet is the fourth season of the Cartoon Network series Infinity Train and serves as a standalone prequel to Book One. It premiered on April 15, 2021, on HBO Max.

Min-Gi and Ryan are childhood best friends who dream of performing in a band and traveling across Canada, together. After a sudden falling out splits them apart, both find themselves aboard the mysterious Infinity Train. Lost and confused, they meet Kez, a free-spirited concierge bell, who accompanies them on their journey. As tensions rise and old wounds come to light, both Min-Gi and Ryan must decide if they can work together or else risk losing themselves forever aboard the Infinity Train.[2]

Book Four was formally revealed through HBO Max's twitter on February 17, 2021.[3]

The Book Four trailer was revealed on Twitter by Entertainment Weekly on March 11, 2021. Here it was also confirmed that Book 4 will be the final season.[4] Owen Dennis also revealed the season's full title in a tweet. Dennis later explained on Twitter this season was not intended to be the final one when it was written (Dennis and the writing staff claim to have enough stories to last eight seasons), and expressed a desire to continue the show if given the opportunity.[5]

HBO continued to advertise Infinity Train. On March 25, 2021 the poster for Book 4 and a more detailed synopsis (the one above) was released on HBO Max's twitter. Then on April 8, 2021 a exclusive clip was released on YouTube.

Cast and Crew


Image Title Premiere Date No. in series No. in season
The Twin Tapes.png "The Twin Tapes" April 15, 2021 (HBO Max) 31 1
Min and Ryan’s lifelong friendship and creative partnership disintegrates.
The Iceberg Car.png "The Iceberg Car" April 15, 2021 (HBO Max) 32 2
Min and Ryan wake up on the train and meet Kez, a talking, floating concierge bell.
The Old West Car.png "The Old West Car" April 15, 2021 (HBO Max) 33 3
Ryan, Min, and Kez get thrown into an Old West jail.
The Pig Baby Car.png "The Pig Baby Car" April 15, 2021 (HBO Max) 34 4
Ryan, Min, and Kez have to make the perfect dessert for a very fussy baby.
The Astro Queue Car.png "The Astro Queue Car" April 15, 2021 (HBO Max) 35 5
The trio sneaks into a guarded nightclub astro tower.
The Party Car.png "The Party Car" April 15, 2021 (HBO Max) 36 6
If they want to move forward, Min and Ryan must play their first show together since splitting up the band.
The Art Gallery Car.png "The Art Gallery Car" April 15, 2021 (HBO Max) 37 7
The gang needs to escape from a haunted art museum.
The Mega Maze Car.png "The Mega Maze Car" April 15, 2021 (HBO Max) 38 8
While they accompany Kez to her home, Min and Ryan air their grievances with each other.
The Castle Car.png "The Castle Car" April 15, 2021 (HBO Max) 39 9
Ryan and Min finally meet Kez’s roommate.
The Train to Nowhere.png "The Train to Nowhere" April 15, 2021 (HBO Max) 40 10
Min and Ryan face a final challenge.



  • Prior to the reveal of season's plot, Owen Dennis joked on Twitter that the fourth book's title was "Demon Slayer", a reference to the anime of the same name and its 2020 film Demon Slayer: Kimetsu no Yaiba the Movie: Mugen Train, which in some territories was known as Demon Slayer: Infinity Train. Dennis claimed it was the show's way of getting revenge on said anime.[6] He later confirmed the official title of "Duet" after the official trailer was released.[7]
  • This is the first season since Book One to not have a main character be a character introduced in a previous season.
  • Owen Dennis believes that Book 4 could be binged or watched at the viewers' own pace. Compared to the previous books, Book 4 works better binged. Ultimately, he doesn't have a preferred way for fans to watch it and hopes people enjoy it.[8]
  • The book is set in the mid- to late 1980s.
    • As such, it is the only book where Grace and Simon do not appear, as they have not yet been born.
    • This is also the only book where Samantha makes no appearances alongside Randall, as they do not meet until "The Beach Car".
    • Ryan uses a payphone to call Min, as smartphones have not been invented yet.
    • A younger Amelia learns how the train works from One-One. Her rise to power is inferred throughout the season.
  • Dennis noted that Book 4 was very much written with the idea of subverting the previous three seasons: they were aware the audience was expecting some form of traumatic event or death, and wished to do the opposite after the dark tones of Book 3.[9][10]
  • Book 5 was planned to be a film documenting Amelia's takeover of the train that took place concurrently with this Book. Had it been made, it would have offered more context for Amelia and One's scenes during this season.
  • This is the only book not to feature Ghoms.