Long distance rail travel is enjoyable if one has the time for it. In the modern world most don’t, and air travel is preferred for it expediency. Long distance rail lines almost always run at a loss paid for by the government. A more practical improvement would be to improve train connections to airports to aid travelers. I would start by connecting JFK, NY with Newark, NJ with one train rather than a bus that gets stuck in traffic, or three different trains as is now the case. That stated, air travel has become miserable due to too much government deregulation of basic traveler comfort and consumer protection.
Time and money both. I loved the few train trips I've taken in the USA, but they cost a lot compared to driving.
Seems to me that rail travel will be almost exclusively for pleasure, so if they can create a good experience and find a way to make the cost attractive, I can see it working for tourist travel. For example, the nearest AMTRAK station to Yellowstone is 5-6 hours away, so how would that work for anyone?