The poem talks about human relationship. It describes people in modern times always carry walls to protect themselves. I think "wall" means a kind of invisible self-defense, and "something" in first stanza means some natural things that have power to confront wall. Spring also means natural things in us and makes us want to break the wall and get free.
I think maybe the effect of the repetition and regularity in form is to tell us to make walls between people is the normal order of thing that everyone should observe. "Good fences make good neighbors," though I don't like it very much, it is necessary sometimes. I think that wall keeps safe distance from others and that makes us feel relieved and won't get hurt easily. However, wall also brings some isolation and solitude which make this world become colder and colder. The reason why we build wall is that human's affection is fragile so we need to build it to protect ourselves. But we also have some enthusiasm that makes us want to get in touch with people without any guard against and just heart to heart. At this time self-contradictory comes out and the speaker begins to doubt that do we really need a wall.
In A Station of the Metro
The poem describes what the speaker saw when he came out of a station car. "The apparition of these faces in the crowd," I think we all have such experience. When we take a sky-train and then come out of it, we can see everyone is in rush steps, and their facial expressions almost the same: no smile and show lifeless. If you stand there for a moment, you will see many faces appear before you and then disappear which is like apparition. You see them just for a transient moment and then you can't remember what they look like as you never see them before.
Apparition is the same, when it disappear, nothing leave. "Petal on a wet, black bough," I think petal means those faces he saw, and those petals seem withered just like those lifeless faces. Maybe "black bough" means the subway, because its color is black, too.