With about 10% overhead on the travel time, a pretty mainline high speed rail line would take about 15 hours to go from DC to San Fransisco. Each train (assuming Japanese trains like Amtrak is buying for Texas) can carry about 1323 seats, plus standing room. They can run 16 trains per line per hour. So, that's 21,168 people per hour passing on the rail.

Assuming the 15 hour lead time (and no loading time of note because Sergents are really good at yelling people onto trains), within 24 hours, the Pentagon could move about 211,680 soldiers coast to cost from time t=0 to t=24 hours. That's coast to coast, mind. If you do it from say the middle of the country to SF, it's only a 4.7 hour trip, so now you can get 19.3 hours of soldiers moving (and arrived) around 408,542 people delivered by t=24hrs. Then, it's another 508,032 every 24 hours after that.

Now, while it's not particularly feasible to have commercially driving HSR across the empty center of the US, the military has a whole different set of priorities, and damn, that's a lot of equipment & people that could be moved really fast. Yes, planes are faster, but there's no way they'll keep up with HSR once the train pipeline fills. This is a latency vs carrying throughput load equation and trains will win it big time. Always have, always will.

The US way way way behind on building infrastructure. Our infrastructure deficit is trillions of dollars, and our transit modality is decades behind Europe, China, India, japan, and even starting to slip behind sections of Africa. We're failing as a developed nation because we refuse to invest in modern transit (and many other issues like healthcare, usurious education costs, and losing our democracy to dictator thinking). We're flailing hard right now. HSR should be a massive investment for our country, along with regional/city rail (trams, metros, heavy regional), but since our population mostly has never ever seen a modern city like Paris, London, Beijing, Tokyo, or Rome so they have no idea what it can even be like to live somewhere well designed for people instead for for cars.

Though, watching the military test the rails by moving a half million people in 24 hours would be hilarious for those of us not trying to coordinate it.

That's not even remotely realistic. It takes a freight train about 5-7 days to travel from coast to coast. It takes a team driving a big rig about 3 days to make the same trip. The reason airplanes make it in hours is because they get to fly over that pesky thing thst slows down trains called mountains