You don't need state intervention to get monopolies; actually it's quite the opposite - economies of scale push for consolidation, and you eventually need government to intervene. Whatever it was known for, the Gilded Era doesn't seem to be associated with large-scale government regulation, but you certainly had mega corps. And given enough leeway, the mega corps will start to take on functions you'd normally associate with government - company tows, stores, and scrip as examples.