Oh, how I hate that term "robber baron". Not only is that concept mostly inaccurate,

Several of them were among this country's most effective anti-monopolists ever.

Standard Oil rose to become a global economic giant by using superior technology to break local monopolies because it could sell better quality merchandise at a lower price - and when it couldn't keep its technological edge (the easy tricks having already been found) it lost its market clout, so that Congress had to rush to pass possibly the worst-written bill in its history to that point so that the government could claim credit for "breaking the Standard Oil monopoly" (actually protecting the remaining local monopolists from competition, forcing their customers to pay more) before it became really obvious that Standard Oil never had a monopoly and the free market was moving it further away from achieving one every day.

"Commodore" Vanderbilt rose to riches by finding loopholes to break *government-protected* monopolies. He started with ferry service across the Hudson River, and ended up having his sets of steamship and rail companies that provided service between New York and San Francisco bought out by a US-government-licensed monopoly provider of that service that otherwise couldn't compete with him... and then he found a new set of loopholes, got back into the same service, and drive the government-licensed monopoly provider out of business. All by providing equal or better service at a lower price.