Roads would never work though. It would take a monopoly to run it if you wanted any semblance of order. Monopolies are inefficient by their very nature.
Power is usually run by free market interests with a monopoly power added for this reason. It's the best option.
Nothing more to say about Amtrac. Someone believes that we can't let the rail industry die (because it would be easier to add to an existing system than start over again...I believe).
It's Econ 101 that monopolies are ineffective. Austrian economics is ridiculed because they won't acknowledge this. They are actually ineffective in 6 ways (a few of which I forget) and none of them require intention. It's easy to understand.
They won't lose costumers, because they've enacted enough barriers to make it effectively impossible for others to enter the market and steal said costumers. They wouldn't be a monopoly otherwise, since someone else would have entered the market and formed an oligopoly.
They don't have competitors to make them have the best product available. Look at Windows Vista. That's in an oligopoly. Imagine that in a true monopoly.
They do things like hiring less effective workers (friends, family, hot girls

) because it doesn't matter as much.
They don't produce at the most efficient price because they don't have to do so. This means higher prices for everyone. If it is deemed to be a good with a high positive externality, then that's a bad thing.
They will also do rent-seeking and waste money for the sole purpose of maintaining that monopoly.
None of this stretches the imagination.
To wrap up the post office...that's point. A reliable postal system is deemed to be necessary. So we have a government subsidized one. The market could fix it, but people would end up paying more in the long run. The positive public externality of everyone having good and cheap postage is greater than any cost (which I actually don't think is too bad...I've heard that they don't run at much of a loss).