The poem appears to be set in some kind of wooded subway in the springtime where there might be ghosts. Weird. The title locates the poem within the metro station, underground. Then, in the first line: an apparition! This is a word that usually hints at the supernatural, especially ghosts. No wonder that when the characters in Harry Potter books teleport from one place to another, it’s called an "Apparition." For this reason, some critics of the poem think the metro station is supposed to suggest a journey to the Underworld, of the kind that occurs in classical epics like Homer’s The Odyssey and Virgil’s Aeneid.
Then, the second line shifts (or in Harry Potter’s terms, Apparates) to a peaceful, Japanese garden. All the people in the metro turn into delicate flowers, and the dirty asphalt they’re standing on turns into a wet tree branch. It’s all very Zen. If you had to ride a subway, this would be the one to pick. Instead of smelling like garbage and stale water, it smells like flowers.