The train itself may be the star of "I love to see it lap the Miles," but we shouldn't forget about the supporting actors – valleys and mountains that serve as the backdrop to the poem. By framing the unnatural creature, the steam train, in natural terms and in a natural setting, Dickinson forces us to contemplate its unnatural nature, if you catch our drift. And, while manmade creation and the natural world coexist uneasily in the poem, there's something missing. Did you notice how, except for the disembodied "I" of the speaker, there are no people to be found? The marked absence of humans immediately tips us off to the fact that the relationship between "man and the natural world" is something we ought to be thinking about.
By playfully comparing the train to a mysterious animal, the poet attempts to frame this new technology in terms of the preexisting relationship of man's dominance over the natural world.
The train's personification in "I like to see it lap the Miles" seems at first to suggest that it is an inhabitant of the natural world; however, the exaggerated nature of these animal-like traits ultimately emphasizes its foreignness to the natural landscape.