In this poem Dickinson describes how she likes to watch trains as they journey along, "lick(ing) the valleys up" as they go by. She gets this message across by personifying the train, many times describing it in language one would usually apply to a horse.
In the first and second stanzas, she says she likes to watch a train as it passes through the valleys, across vast expanses of land, stopping to "feed" at tanks to get the fuel it needs to continue along the way. The train then shows its great size and power by "prodigious(ly) step(ping)," or traveling, around mountains, and "superciliously," or rather proudly and condescendingly, peering into the shanties it passes. It also at times must a "quarry pare," or go through a tunnel.
When it does this, the third stanza explains, it must "fit its sides, and crawl between" the narrow walls of the tunnel, sounding its horn "in horrid, hooting stanza" to warn any people who may be near that it is coming. It then comes out of the tunnel and proceeds to roar down the hill ...
In the fourth stanza, this racing, noisy train - "neigh(ing) like Boanerges," or rather shouting like extremely passionate preachers - suddenly and punctually stops, quiet and all-powerful, at the train station - its "stable door."