If you were a person living in Paris near the beginning of the 20th century, there would be a lot of reasons to be afraid of the metro. The crowd of anonymous strangers pushing past one another, blank stares, the dirty wet ground. This poem, though, presents the new technology as the scene of a mystical experience, in which the poet and his readers are reminded of the serenity and calm of a Japanese garden.
The poem does not have an opinion one way or the other toward modernity. The poem treats our relation to technology as essentially the same as our relation to nature.