The poem uses end rhyme, when the words at the ends of lines rhyme, which produces a kind of predictability. The rhyme scheme is abab cdcd efef. This creates a certain music to the poem because end rhyme allows us to anticipate sounds, just like we anticipate sounds in a musical piece. If a piece of music has played a certain refrain over and over, and then, one time, it does not hit the last note, we would notice. End rhyme produces the same kind of effect.
The rhythm in this poem is a bit trickier. The first line of each stanza is in regular iambic tetrameter (four feet, each foot with one unstressed syllable followed by one stressed), and this makes us expect to hear a similar, regular rhythm from the next lines, but Millay changes it up. Line two, "And the day is loud with voices speaking," begins with an anapest (a foot with two unstressed syllables followed by one stressed) and ends with an extrametrical syllable (an extra unstressed syllable at the end of a line of verse). The third line also begins with an anapest but lacks the extrametrical syllable, and the fourth line is regular iambic tetrameter with an extrametrical syllable. Millay continues to make metrical substitutions and employ extrametrical syllables throughout the poem.
The end rhyme, then, would seem to add some predictability to the music of the poem, but then the rhythm of the poem actually detracts from that sense of predictability. Perhaps she meant the skewed rhythms of some lines to mimic the movements of the "train[s] [that go] by all day" and "All night" (lines 3, 4). Perhaps each stanza is meant to begin in predictability, as a life with that is "warm with friends" would be, but that predictable life is broken up by the unpredictability of the trains themselves as well as the idea of getting on a train, "No matter where it's going" (9, 12).