In my view the main theme would be: Progress as the new natural.
Indeed, the symbolism the author uses for this poem is that of a domesticated animal, more specifically a horse. It is no coincidence that Native Americans called trains “the iron horse”. The train is described in very equine terms: “to lap the miles, to lick the valleys up, to feed itself on tanks (like a hose that drinks from a trough), and stopping at its own stable door”.
A horse is an animal from nature and a train is not, it is a pure human invention. However, Dickinson juxtaposes very oxymoronic concepts by referring to it as a horse. Since nature and technology are juxtaposed in this manner, it can be inferred that for the author, a train, which represents technological progress, is the new natural state of humanity.