You asked a legitimate question, and also came to a conclusion. Your conclusion is easy to refute, hence you got most replies there. I'll focus on your question

You asked a legitimate question, and also came to a conclusion. Your conclusion is easy to refute, hence you got most replies there. I'll focus on your question. The short answer is "everything". Let's start either public desire. Nobody wants a 300mph train to go through their neighborhood. Especially a train they'll likely never ride. One that doesn't stop locally. Since public interactions are an important part of any major project in the US this has a real cost implication. Equally public perception en masse leads to the project becoming political, so it can't fly under the radar. (It benefits at least some politicians to be anti so they're anti regardless of merit.) Get past the people, and you get into the environment. Long distance trains cover a Lot of land so lots and lots on environmental impact studies to do. So you're spending mountains of cash before you can even be assured the project will happen. Because its such high risk, that cash comes very expensive. (Politically expensive if its public funds, commercially expensive if its loans or investments. ) If you got this far, construction is expensive because you're not just "laying track". Everything you touch will be subject to a million local regulations, which will differ from one county to the next. You will need an army of lawyers, inspectors, endless tweaks here and there, and so on. The trains will be expensive. Any craft travelling at 300mph will necessarily be expensive. You'll want it to be safe, which means (quite reasonably) this is not an area to be cheap. You specified maglev, but that is almost the least interesting part. All the above applies to any rail project. Maglev adds to the cost primarily in terms of tech, and the results of tech failing. (The train would need to safely transition between maglev and not maglev, at 300mph). So, to answer your question, you need to ignore the public, ignore the environment, ignore the effect of public opinion on politicians, ignore local regulations, be able to aquire land (in a straight line) without hindrance, and of course source a cheap train. And at the end of it? Maybe you'll have a train people will ride. Maybe you won't. (For reasons mentioned elsewhere in this thread, I'm guessing won't).