4 ideas
Several assassins board the same Tōhoku Shinkansen from Tokyo to Morioka on separate jobs: Kimura hunting the schoolboy who hurt his son, the partners Tangerine and Lemon escorting a crime boss's son and a suitcase of money, and Nanao sent to steal that suitcase and get off at Ueno. Because these assignments share one train and one suitcase, they keep crossing and colliding.
The Prince uses the assumption that a polite middle-schooler is harmless, along with threats against Kimura's hospitalized son Wataru, to control adults who should be more dangerous than he is. His question 'Why is it wrong to kill people?' serves him as a tool for unsettling others, and the book does not let it stand unanswered, giving Suzuki a reply to it.
Nanao assumes any simple job will go wrong, so he has a contingency and a quick improvised response ready for each escalating accident. Expecting misfortune makes him the most adaptable killer on the train and the one who survives. Under this lens, a person's sense of being unlucky is a training regime that builds resilience, not only a burden.
The shinkansen's fixed schedule and short stops limit when characters can get off, which keeps them running into each other in the same few carriages. That confinement helps turn small mistakes, like a misplaced suitcase or a body in a toilet, into escalating trouble, though chance also plays a part, as when the Wolf turns up at Nanao's exit.