This poem contains both alliteration and consonance, two related literary devices that allow its short form to link disparate images with sound. There are only a few instances of each, and they all occur in the poem's second line. Alliteration exists in the /b/ sound shared by "black" and "bough," while consonance exists in the /t/ sound shared by "Petals" and "wet" as well as in the /l/ sound shared by "Petals" and "black." In a poem this sort, the use of so many repeated sounds is no coincidence, but rather a deliberate choice on the part of the speaker.
For one thing, the insistence on the /l/ sound slows down the second line. Whereas in the first line the people's faces rush by so quickly that they seem like mere apparitions, in the second line it feels as though the speaker is really savoring this image of "petals on a wet black bough." The luxuriousness of the /l/ sound causes time to pause, in a way, as if the speaker has pressed a slow-motion button on the image before them.
These sonic resonances stand out not just for their appealing sound, but also for the fact that one line contains them while another does not. The lack of similarly resonant sonic devices in the first line might make the image of the crowded train station seem less beautiful or coordinated than the second line, which, by contrast, consists almost entirely of words that all sonically resonate with one another.
This disparity matches each line's content, the first portraying an image that is, after all, less beautiful and more hectic by representing a crowded metro station, while the second line shows a relatively orderly and peaceful glimpse into the natural world.