Now take the route from Flagstaff, Arizona, to Los Angeles. BNSF is the only one who serves that route, because they're the only railroad that has tracks to Flagstaff. So BNSF has a monopoly on railroad service to Flagstaff.
But the fact is, there's not enough railroad traffic that originates in Flagstaff to make it economically worthwhile to have two different rail lines.
And in fact, this kind of "monopoly" has always been an issue with rail service. If you have a factory in Chicago, there were something like 18 different railroads that served Chicago back in the day, but only one of them had a track to your door. So you only had one choice for your factory, even though there were 18 railroads in town.
On the other hand: That railroad had a monopoly on your rail service because they built a track to your door (at their expense), and the other 17 railroads didn't. So I have a hard time getting too upset about this kind of "monopoly" as any kind of unfairness.
The fact is, though, the main railroad competition is trucks, not other railroads. (Also barges, for low-speed bulk stuff.) And the trucks (and barges) are running on highways (and waterways) paid for with taxes (including the truck's gasoline taxes), whereas the railroads are paying for their track (and paying taxes on it).