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 unnatural 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.