In order to be useful trains need to form large network. Few mile subway route is not so interesting. A tunnel for cars is different. Even a single and relativ

In order to be useful trains need to form large network. Few mile subway route is not so interesting. A tunnel for cars is different. Even a single and relatively short tunnel is useful and people would probably pay something for access. This means you would start making money on the investment quite quickly. I agree that trains are the right thing to do, but that is much larger project and likely too big to be an option for private company.