Answer:
I suppose I'm a bit late to answer this question since it was posted 5 days ago, but I believe it is: The phrase stable door suggests the poem was written when horses were used as transportation.
Step-by-step explanation:
The entire poem revolves around a train and how facinated the author is with them and their speed. It's safe to assume that, since they are comparing the trains to horses, that means that these two are the most common transportation methods of that time. Thus leading me to the conclusion that choice D is the correct answer. I hope this helps anyone in the future. :-)