And then — prodigious step
Around a Pile of Mountains — (4-5)
The speaker depicts the train on a scale larger and more powerful than a mountain range – pretty heavy duty. It's described as "prodigious," and it is clearly something of a prodigy, as it's able to lightly "step" around a "pile" of mountains like a child stepping carelessly around a pile of toys.
And supercilious peer In Shanties — by the sides of Roads — (6-7)
If you had any doubts about the train's strength relative to our own, let them be erased here. The train clearly views itself as superior as it passes by human habitations, a sentiment betrayed by the adjective "supercilious."
And then a Quarry pare (8)
As if the train didn't seem powerful enough already, the speaker demonstrates her awe of its physical strength here – it easily "pares," or slices, through rock as it passes through a tunnel.