“In a Station of the Metro” is about imagery as the speaker sees the mass of people in a subway station petals of a tree branch are imagined.
The faces are a sensed presence of identities that have come to help this traveler. The metro suggest the travel that was required, and the faces suggest the help that comes.
The question we are left with is whether it is really help that came. The faces are wet, more like petals than a life form, and have a blackness to them and are more like tree branches than humans.