I like to see it lap the Miles—And lick the valleys up (1-2)
This seems like a pretty clear and simple expression of admiration – the speaker "likes" watching the train as it passes through the countryside. However, the likeability of the train soon becomes questionable…
And then — prodigious step Around a Pile of Mountains — (4-5)
The speaker seems to be in awe of the ease with which the train navigates these impassive features of the landscape.
Complaining all the while In horrid — hooting stanza — (11-12)
Hmm. Is this admirable or not? After all, nobody likes a crybaby. The speaker seems truly ambivalent towards the train at this time – "horrid – hooting stanza" is a pretty negative description. Still, even though it's negative, it's also fascinatingly evocative; we wonder what it must sound like. Even when the train's not looking (or sounding) so wonderful, it has a strange appeal.