설국열차 (seol-guk-yeol-cha) literally means "snow country train," but it's also the Korean title for the famous Bong Joon-ho film, Snowpiercer. It's a powerful and unique word you won't find in most songs.
In "Spring Day," the lyrics describe being on the "설국열차 left alone," using the train as a metaphor for a lonely, endless journey through the emotional "winter" of missing someone. It vividly paints a picture of being stuck in time, waiting for the metaphorical "spring day" when they can reunite.
Right from the opening line, BTS hits you with "보고 싶다" (I miss you), admitting that saying the words aloud only makes the ache worse. The whole track treats emotional distance like endless bad weather. Winter refuses to leave—they even joke grimly that it feels like December in August ("팔월에도 겨울이 와" [winter comes even in August]). To describe that cold, isolated feeling, the lyrics drop a direct pop culture reference: "홀로 남은 설국열차" (a Snowpiercer left all alone), borrowing the frozen, runaway train from Bong Joon-ho's famous film to describe a mind racing ahead through the ice while trapped in place.
Yet despite all the talk of trying to blow the memory away like smoke, the song refuses to stay completely frozen. There is an honest back-and-forth about who changed and who drifted away, but it always circles back to patience. The narrator promises that no darkness or season lasts forever, watching for the cherry blossoms to bloom ("벚꽃이 피나봐요" [cherry blossoms seem to be blooming]) and asking their best friend to hold on just a little longer until the cold breaks and they can reunite.