It's not as exclusive as you suggest it is. Contrail, owned by the Federal government, owned several thousand miles of rail corridors. State governments own hundreds of miles of rail corridors. On rail corridors owned by government, government can dictate what occurs on it.
Oklahoma, within the last decade, sold a railroad corridor between Oklahoma City and Tulsa to a private railroad, contained within the sale contract was allowing passenger rail operations in the future by just about anyone. Oklahoma bought the rail corridor for a song when an existing freight railroad company abandoned it. On rail corridors once owned by government, government can dictate what occurs on it in the future.
North Carolina encourage the building of the NCRR at the very beginning, instead of providing land grants it bought stock. A hundred years later, the private railroad company went bankrupt, and the state took up its option to buy the rest of the shares at a song, and now completely owns it. There are many ways for government to own a railroad corridor.
Illinois stepped in when one private railroad company bought out another, during the sale court processes to prevent monopolies, had the new larger railroad company agree to higher speeds passenger trains on a corridor Amtrak was already using. So, even without corridor ownership, Federal and State governments can open up passenger train access to rail corridors, under the right circumstances.
Local transit agencies have bought railroad corridors form willing railroad companies too. Whereas I don't believe any government bought a railroad corridor from an unwilling railroad company, there are opportunities to do so when previous unwilling companies become willing.
Railroad company mergers and bankruptcies have occurred since the first decade of their existence. Railroads companies have been abandoning rail corridors as well. Opportunities do arise more often than many of us think.