The utopian vision is that the entity with a monopoly on violence would ever produce impartial, socially good outcomes. Who are these regulators that would do a good job and who are these politicians that would be okay with them doing that? The politician's incentive is to get repeatedly elected which means money from donors and promising voters blah-blah-blah though those voters rarely hold them to account for their promised visions failing to materialize.
The vision of a society dedicated to free association of people should be a guiding star. The history of companies, even with a largely captured government, is that they tend to continually change, often growing smaller. Many of the big names in business nowadays did not even exist 50 years ago. Many of the big names of previous eras have long gone away. Competition yields temporary winners, but in a dynamically changing world open to all to be a competitor, they do not stay winners forever. Rockefeller kept trying to buy out all the refineries, but people can't making more. Eventually, he gave up. Cornering a market is very hard if government is not putting up barriers to entry.
As for authoritarian companies, more freedom for the working class to form their own businesses is the response. Our current government makes that very hard with a few exceptions, such as the seemingly entirely unregulated lawn businesses which is filled with small independents. Get rid of the government overhead, allow people to just do stuff for others as the others wish and get paid as mutually agreed, and then there would be competitive pressure on companies to change their approaches.
And, if not, make their replacements. Make democratically run companies. They exist and if that is what workers want, then they might thrive.
As for rail systems, there are certainly other means of transportation. That can serve as a temporary work around for a bad rail operator. Meanwhile, if there is sufficient need, one could construct new rail routes to go around the old ones if need be. It would start small, doing the most profitable portions and then work out from there as needed. Just the threat of that would probably lead to sensible negotiations.
Of course, to address the government supported oligopoly, one could also use eminent domain to take the rail tracks and then give them to the local communities in some kind of newly formed, broadly and locally owned companies. One would need to come up with ways of dispersing government property fairly anyway and one could also, at that time, redistribute any critical privately held infrastructure which had been supported unfairly by the government.