In describing the train as "licking" the valleys, the speaker transforms the train into an animate being (and a slobbery one, it sounds like). This is a great example of transformation appearing through the imagery in the poem.
And then, prodigious, step
Around a Pile of Mountains — And supercilious peer In Shanties — by the sides of Roads — (4-7)
Now we see the transformation of the landscape. The train moves from the great natural outdoors toward "Shanties" and roads. These lines show us how America was being transformed at the time that Dickinson was writing. Society was becoming industrialized… even if the shantytowns weren't quite as impressive as the Big Apple's skyline.
And then a Quarry pare
To fit its Ribs And crawl between (8-10)
The train makes its way through rocks and mountains, "crawling" between them. The fact that trains could cross through such difficult landscapes was a new and exciting thing back in Dickinson's day. Not only do we see the impressive train here, but "a Quarry pare" also gives us an image of the transforming landscape.