Freed from the tunnel, the train eagerly goes downhill. This line has a light, playful quality. (We can't help but picture a cute puppy chasing its tail.)
It's also like a riddle: What can chase itself?
Answer: a train made up of multiple cars, each one following the engine down the hill.
And neigh like Boanerges —
The creature lets out a rumbling cry, or "neigh," which reminds us of its horse-like qualities from the first lines.
The speaker compares it to Boanerges, a Biblical name that means "son of thunder," and generally refers to a booming, loud preacher or public speaker.
Then — punctual as a Star Stop — docile and omnipotent At its own stable door —
In these lines the speaker uses a simile. She says that the train is "punctual as a star."
Stars, of course, show up in the sky at a specific time each night. It sounds like the speaker is suggesting that the train is as punctual as nature.
At last, the creature stops, right on time, and placidly returns to its home, or "stable" (another horse reference to bear in mind).
In these final lines, the speaker also describes the train as "docile and omnipotent." This characterization again gives humanlike (or even super-humanlike) qualities to the train. On the one hand, the train is "docile," or submissive. On the other hand, it's "omnipotent," or all-powerful.
The train, in other words, is a complicated thing with many qualities and many characteristics. And it's all pretty new to the speaker, so maybe she's still not quite sure what to make of it.