Cost is probably off by an order of magnitude, if not more. Why? Unions and other groups. Building anything in this country costs massively more (and takes significantly longer) than one could imagine because of our unions are not business symbiotic. The goal of union leadership is to extract as much as they can out of the businesses they infect, even if this means their demise.
A lot of the unions that would be part of such a project have some of the laziest and most problematic people working for them. If you come from the Silicon Valley tech world my words make no sense to you. In fact, you might think I am nuts. All I have to say is: Do a good size trade show exhibit at a few of the unionized convention centers in the US and then see what you think about US unions. Then repeat that experience at various locations in Europe and Japan and see the difference. I have done just that. US labor unions are destroying our country from the inside out.
How many significant new civil engineering projects in the US can you name over the last, say, fifty years. Right.
Hyperloop cost would be way more than this paper seems to predict.
The political factor is grossly underestimated. Our reality is that we live at a time of political deadlock. Nobody can or wants to make a decision and the decisions we do make tend to be suboptimal, sometimes grotesquely so: example, California high speed rail.
Finally, it addresses the wrong market. I am not sure why people insist on applying trains to transporting people in the US. Sorry to resort to reality folks: If you build it they will NOT come. We do not have that culture and you will not inspire it simply by building trains. You'd have to forcefully push people in that direction through legislation that would make it too expensive to not use rail. In other words you'd have to declare war against other methods of transportation through punitive actions.
The right place for high speed rail in the US is cargo, not passenger rail. If we could evolve our cargo rail systems to move at 300 km/hr rather than 50 the consequences would provide economic benefits and development beyond a century.
The final point is related to politics. We have a decision making system that allows anyone to vote. And, while this is commendable, it does create horrible problems. Imagine allowing a random group of, say, ten people deciding whether or not your child should have surgery. I'd be surprised if anyone thought this to be a good idea. No, most people would rather have a group of experts in the field, more than likely MD's in this case, vote for such a decision. The ridiculous California high speed rail project is a result of hordes of low-information, mathematically challenged, technologically ignorant and financially ignorant voters being led by the nose by unions, media and political forces.
How do you move forward when people like that can vote on these issues and their vote has equal value to that of an expert in the relevant fields: a PhD in Physics, an engineer, a financial expert, etc.
Regrettably this is not a technological problem. It is far more complex than that.