Symbolism, Imagery, Allegory
This one is from the narrator and runs throughout the novel: the idea that the ugly, square, fact-based, oppressive mills look like fairy palaces with elephants in them when they are lit up at night. The image first pops up as something a person riding by Coketown in a fast-moving train might say – in other words, someone who doesn't know any better what the reality of the place actually is. It's an idea dripping with irony, since we already know that there is nothing beautiful or magical about the factories. Then, in a pretty neat trick, "Fairy Palaces" becomes kind of a nickname for the mills, and is used whenever Dickens needs to poke readers awake and yet again quickly remind them how awful life is for the factory workers.