Natural monopoly in the formal macroeconomic meaning of very high fixed cost with virtually zero marginal cost (ex. it costs ~nothing to transmit an extra watt of power, fly one more passenger, insure one more person, etc). In these cases basic econometric theory shows that production cost is lowest with a single provider, adding two or more competitors increases total cost of production without changing the total output. Obviously nothing obtains these conditions precisely in the real world, but in general the principal holds for big infra stuff. I think your example of a bus system faces the obvious fact that the US dumped a ton of resources into private auto travel which makes it challenging for any public/shared transit to succeed.
The power grid is another classic example of a natural monopoly, it costs a lot to make a power plant and hang a bunch of wires, but once it's running it's basically free to send a small unit of additional power to a person. For this reason the US was going to nationalize the grid back in the 20s but at the last minute decided to lease to a "regulated monopoly" private corp (IIRC it was some dude in Chicago who convinced them to give him the lease). Texas tried to roll this back in the like the 90s and their power costs shot through the roof and still remain nationally high.
The power grid is another classic example of a natural monopoly, it costs a lot to make a power plant and hang a bunch of wires, but once it's running it's basically free to send a small unit of additional power to a person. For this reason the US was going to nationalize the grid back in the 20s but at the last minute decided to lease to a "regulated monopoly" private corp (IIRC it was some dude in Chicago who convinced them to give him the lease). Texas tried to roll this back in the like the 90s and their power costs shot through the roof and still remain nationally high.