A bunch of years back, I remember reading about 500 miles as a rule of thumb for that tradeoff. Between two cities less than 500 miles apart, high speed rail could be the preferred travel choice, while air travel clearly wins for longer distances.
Obviously the exact distance depends on the details, but we would do well to present high speed rail for the trips that it can be better.
For me personally, I love travel by train and hope some day to travel long distance at least once. I live near Boston, one of the few US cities with pretty good transit, and one end of Acela, the closest we have to high speed rail. From the time Acela opened, it was immediately the best choice to travel Boston —> NYC. However I’ve been to DC every year and never tempted to take the train. Flying is better for that distance, given how slow Acela is: sure enough, close to 500 miles