I can see high speed trains being feasible on some routes in the US, but not everywhere. They could work along the Pacific coast and in the eastern states where the population is more dense, but from the Mississippi to the Sierra Nevada mountains, I just can't see it working. The cities are just too far apart to justify building the infrastructure that would be required. I'll use Salt Lake City as an example, because that's the area I'm most familiar with. The nearest cities to which routes might be built are Denver, CO; Phoenix, AZ; Las Vegas, NV; Boise, ID; and Sacramento, CA. The shortest distance of those is 350 miles to Boise, ID. There wouldn't be a lot of demand for that route, except as a stopping point on the way to Portland, OR or Seattle, WA (each another ~500 miles). It's 400 miles to Las Vegas, 500 miles to Denver, and 650 to either Phoenix or Sacramento. Most often, when someone travels by plane from SLC to any of those cities, or the other way, they are on their way to somewhere else, so those routes would really be just parts of longer routes elsewhere. Let's look at what might be a popular route, Chicago to San Francisco. That's a trip of well over 2000 miles. A high-speed train would probably stop at maybe Des Moines, IA; Denver, CO, Salt Lake City, UT; possibly Reno, NV; Sacramento, CA; and finally San Francisco. That's 4-5 stops in 2000+ miles, and there wouldn't be many people getting on or off at those other stops. That's about the same distance as a trip from Warsaw, Poland to Lisbon, Portugal. I dare say there would be more than 4-5 stops on that trip, and a lot more people making shorter trips that are part of that longer route.
Furthermore, one can make that trip (Chicago to San Francisco) for about $150 in 5 hours on a plane. I doubt a high speed train would be any cheaper and would take at least twice as long.
Another issue is the mountains and weather. A high speed train route through the Rocky Mountains and through the Sierra Nevadas would be plagued by snow problems in the winter time, not to mention the difficulty associated with building the lines through the rugged terrain. When storms that dump 4 feet of snow on 100 miles of tracks at 7000 feet elevation happen several times every winter, it's going to be a challenge to keep the tracks clear enough for a train moving at 200+ mph.
I think that people who wish for high-speed trains and say they would work in the western US have either never flown over that area or didn't pay much attention when they did. Take it from one who has lived there for many years, the intermountain west is very sparsely populated. These states have about the same population density as Canada (you know, about the same population as Tokyo spread out over the second largest country in the world). That's about 1-2% of the population density of Western Europe or China, where high-speed trains actually work. The population of eastern states is similar to Europe, so trains could be a very good idea there. The problem with trains is that the amount of infrastructure required is directly proportional to the distances between the population centers. They work where those population centers are close, but they become much more expensive as those areas get further apart, and as the terrain between them becomes more rugged. With air travel, distances and terrain between population centers don't matter when building infrastructure.