Even with precheck, one still has to show up at the airport earlier than a train, since boarding stops 10-15 minutes prior to departure, TSA adds at least five

Even with precheck, one still has to show up at the airport earlier than a train, since boarding stops 10-15 minutes prior to departure, TSA adds at least five to ten minutes, and the terminal topology of large airports requires a long walk from the curb to the gate. For trains, even large stations don't have that long a walk, since the tracks run parallel to one another. As long as one shows up before the train pulls into the station, it's all good. As you point out, trains definitely don't replace all flights, but for some dense corridors in the US (eg. Washington-Philadelphia-NYC-Boston, Dallas-Houston, San Francisco-San Jose-Los Angeles-San Diego) high speed rail would offer a better experience (larger seats, more comfortable ride, more uninterrupted time for working) and a comparable door to door time.