One of the central mysteries of the poem is, why are the faces the poets sees an "apparition"? This word usually refers to ghosts or supernatural spirits. Pound seems to be comparing the beautiful strangers in the subway to ghosts who appear suddenly and then disappear from your life just as fast. You catch a glimpse and that’s it. Plus, the subway is underground, which could make a reader think of epics like the Aeneid, the The Odyssey, and the Inferno, which all include journeys to the Underworld.
The subway does not represent the Underworld because it would not fit with the peaceful natural imagery of the second line.