The world's busiest airport revealed
Despite just 0.3 billion population in the United States, the reason why some airports in the United States are ranked in the busiest airports is that US citizens feel free to use airplanes and have actually no choice but to use the transportation when moving longer than hundreds of kilometers. On the other hand, along North-East Corridor, Amtrak rail networks can be alternative transportation. But, it's hard to say they are main transportation mode, because they cannot offer competitive fare against airplanes.
Without interference with railway companies by the government, it would be impossible to cause modal shift to railways.