無限列車 — 無限 (infinite / boundless) + 列車 (passenger train) — names both a physical Taishō-era steam train and the dream-trap that the demon Enmu uses to convert that train into an inescapable loop. The arc 無限列車編 (Mugen Train Arc) covers Volumes 7-8 of the manga and was adapted into the highest-grossing Japanese film ever made when released in 2020. For an international audience this is the entry point: many English readers met Demon Slayer through Mugen Train's theatrical release, which is why the term has stuck untranslated in the English-language fandom.

The train itself is iconography: a Western-styled Taishō express with a black locomotive, brass fittings, and the kind of plush red-velvet first-class car that signified modern Japan to its 1920s passengers. Inside that car, Kyojuro Rengoku is found eating a gyū-nabe bento — a moment that compresses the entire Taishō contradiction into one image (modern train, modern beef, pre-modern warrior on it). The film's climactic battle and Rengoku's death scene occur after the train has been physically derailed; the title refers as much to Enmu's dream-loop as to the train's name.