The cost problem for medium or long distance trains is the cost of human labor.

In a given 10 hour work day (apparently common for airline flight attendant) how many flights can that worker work? Let say New York to Los Angeles flights. So the answer is about 2 flights per day. Compare that to the time it takes by train for the same distance, which is about 72 hours. Because of this length this means you also have to have more than one set of crew available to the train passengers.

The staff have to be paid significantly longer on the train to transport far fewer people simply because of more elapsed time. It may be worth it for a nation to subsidize long distance train travel, but understand that that is the problem with profitability vs airplanes that can simply move more people in less time, requiring few paid human hours of labor.