This is sometimes posed as a question in elementary topology. Trains and cars are linear operators constrained to one dimension. Travel within a city or country is constrained to two dimensions. Travel then becomes an areal operation. The only way to effect a transform with the linear operation is with a grid, which means inefficiency and bloated costs when applying an engineering solution. Those inefficiencies manifest as either a lot of nodes on the grid, or limiting nodes which means larger gaps between nodes which have to be covered means. Air travel avoids some of this by adding a third dimension, which makes point-to-point travel possible, at least on a limited basis. You still need nodes (airports), but the former linear gridlines are now stretched and warped to minimize node to node travel. The ideal solution is point-to-point air travel using a personal conveyance, and while we have been waiting a long time for the George Jetson-type flyer, I'm not convinced I will ever live to see it.
Which reminds me of the old jest about topologists:
Q: What is a topologist?
A: Someone who cannot distinguish between a doughnut and a coffee cup.
The ideal solution is point-to-point air travel using a personal conveyance, and while we have been waiting a long time for the George Jetson-type flyer, I'm not convinced I will ever live to see it. Politicians will never allow it because someone might use such a conveyance to kill a politician. If cars were a new invention, we would never be allowed anywhere near them.
The USA's well-developed aviation system simply renders high speed rail irrelevant. The only place it might work is if you built it from Las Vegas to somewhere near L.A. - moving Baker and Barstow, CA residents out of the way comes with less political cost.