I like to see it lap the Miles — And lick the Valleys up — (1-2)
While it's cool to think about the train gobbling up the hills and valleys as it goes, it's also kind of creepy and menacing. This image of ravenous eating doesn't bode well for the natural landscape.
[…] prodigious step
Around a Pile of Mountains — 5
This image dwarfs both man and nature in relation to the train. The word "pile" trivializes the mountains and makes us feel our own smallness and weakness; if we're already teeny tiny and insignificant next to a mountain, how can we possibly compare with the power and majesty of the train?
And then a Quarry pare
To fit its Ribs And crawl between (8-10)
This tunneling action of the train seems to emphasize the newfound, technologically-assisted might of man; where rock might once have stopped us, we can now pass easily.
In horrid — hooting stanza — (12)
This phrase brings to mind a wild, untamed animal, yet the train emits this sound as it's doing something very un-animal-like (plowing through a rocky impasse). This is another natural image that's manipulated to demonstrate just how very <em>unnatural</em> this beast is.
Stop — docile and omnipotent At its own stable door — (16-17)
The image of the stable (and the "neighing" of the train in line 14) ultimately reminds us that it, like the horse, is a domesticated creature. Man still wins out over both nature and technology…for now, at least.