The apparition of these faces in the crowd;
Petals on a wet, black bough.
I think the metro that Pound is referring to is a literal metro where the trains come and go. It seems as if Pound is allowing us into his mind and thoughts. The two lines, separated by a semicolon, bounce from one though to a completely unrelated second. The word “apparition"literally means a ghostly figure. Ezra’s view of the crowd of faces could be described as beautiful and unique.