This is an intriguing image of the train being refueled. Interestingly, the speaker doesn't describe the train being fed by human hands, but instead gives this amazing technology a kind of autonomy or agency.
And supercilious peer In Shanties — by the sides of Roads — (6-7)
Oh dear. Feeling high and mighty, are we? The train's inner life is filled out more here; it seems to feel a kind of superiority to the inhabitants of the shanties (humans). What might this imply about the relationship of technology to its inventors?
And then a Quarry pare
To fit its Ribs And crawl between (8-10)
Here, we see that technology is able to surmount even the greatest natural boundaries – it carves paths where none existed before.
Then chase itself down Hill — (13)
This line also implies that the train has a measure of independence and a mind of its own.
[…] docile and omnipotent (16)
Yeah, yeah, yeah, we know. We've brought this quote up for every theme. It's for good reason – this is kind of the crux of the poem. The train is "docile," and does what it's supposed to do. However, it (and other new, mighty technologies like it) is also "omnipotent," which implies that it's more powerful than we ourselves are. That's a little creepy.