It's original title in Japan is Maria Beetle, and the story — both in print and on screen this July — is about five assassins (with Brad Pitt as the one we're supposed to side with) on the same bullet train from Tokyo to Morioka, all after the same goal. Let your imagination run wild with that and just conjure up images of the chaos delviered by Leitch, who also directed Deadpool 2.
In an interview with Crime Fiction Lover, Iraska explained his inspiration to write the novel in the first place. "When I completed Maria Beetle, I did it with the thought that I wanted an emphasis on entertainment. I wanted to create something exciting. I wanted to write a novel more exciting than not just other books, but also movies and manga. I’d previously released Grasshopper, which can be read as a prequel to Maria Beetle but is tonally very different, so I feel that Japanese readers enjoyed that difference. In Japan, readers read Maria Beetle with knowledge of my other works, but an English-speaking reader would read Bullet Train without any prior knowledge, so I think the experience might be different."
As to what separates Bullet Train from other thrillers, he opines, "In general, I feel that in action thrillers you have the main characters going against, for example, the mafia, corrupt politicians, murderers, etc. Of course, I like this kind of movie or novel, but when it comes to writing myself, I want to try to present a different kind of evil, a different enemy. In Bullet Train, this is the boy called Prince, who could be read as a metaphor for the evil desire to subjugate others. The other characters don’t necessarily battle against this evil desire but are continuously at its mercy. I think this type of dynamic, where there’s no simple juxtaposition of good against evil, may be characteristic of my writing."