for short to medium distances it is:
1. Much more comfortable (wider seats, more luggage space, etc)
2. A lot more convenient. No taking off shoes, check points and lines. Plus the chances are a train station would be a lot closer to the center of the city, rather than an airport.
3. Cheaper and less pollution.
E.g. If the CA High Speed Rail got build, it would take about 2.5 hrs. from downtown SF, to downtown LA. By plane it is about 50mins. BUT, you should add the time it takes for check in check out (at least 45mins), then the time to travel to the airport themselves (30 mins with Bart, if you live in SF), etc. If High Speed Rail would have been implemented throughout most of the major cities of US, It just wouldn't make sense to fly for short and medium distances.
Coast to Coast, yes, coast to Midwest yes, SF to LA or LV, probably not.