Why are the machines in the factory compared to elephants? Is Dickens connecting the factory to the circus?

The narrator mentions this motif quite allot. It runs along the idea that the "ugly", square, fact-based, oppressive mills look like circuses with elephants in them when they are lit up at night. The reality, of course, is that they are places with harsh near slave-like conditions. Unlike circuses which light up the imagination and innocence of children, these factories steal them. They are soul crushing places that, when lit up at night, are dripping with situational irony.