I know that people built and use the Metro is because that time is important for them. They wouldn't want to waste time on commuting; also, because the population is getting bigger, and that makes the traffic busy, therefore, for both reasons that something as Metro has been invented. Because this poem is talking about the modern world, and in modern society, everybody loves to be on quick paces: they talk fast, walk fast, drive fast, and appreciate fast things (that is why fast food restaurants are so popular now), that is why the poet wrote the poem so short-if he write the poem too long, people wouldn't want to read it, also, it wouldn't fit in what he is writing about: the fast, modern world.
"The apparition of these faces in the crowd"
It describes the typical image of the Metro station: the train comes and gradually stops by the platform, countless people try to get in the already crowded train. Then very soon the bell goes and the sliding doors shut and off the train go again. Suppose we are standing by the platform watching this happening. When the train accelerates its speed, all we can see, from the windows, are each person's heads, their faces. Because the train goes to a top speed in seconds, it is as if we are looking at things that vanishes very quickly, like the apparitions. We could also imagine the scene as the crowd is in the station. Everyone walks past you are all in fast paces, and before you had time to look at them properly at their faces, they are gone, disappeared into another big crowd of people.
"Petals on a wet, black, bough"
Petals makes us think of something gentle, fragile; bough is on the opposite side: it stands firm and strong. Petals are like us humans, and bough is like the Metro. Humans can be hurt physically easily by the steel-made trains. Also, wet and black gives me a feeling that the bough is kind of old, rotten. Maybe the Metro in Paris had a long history and the trains had grown old with time. But still, people (the Petals) are still travelling by it. It might be old but those who use it as their daily transportation cannot live without it.
The tone is critical. Words that are used in the poem doesn't sound positive, and it makes the readers feel uncomfortable, such as "apparition", "crowd", "wet", "black". It might be the poet's feeling toward the Metro, and he wrote the poem in this way to make us to have the same feeling as him, too.