If flights are dirt cheap

HSR is generally more convenient and less expensive than air travel, thanks to the ability to run trains directly out of the center of major cities while airports are flung to the outskirts. If you've got the choice between a $30 plane + hours of driving and TSA and boarding and landing, or a straight shot to your train station and then $30 to pop on the next one that's scheduled to leave, its an easy choice.

it’s about the amount (& type) of energy used to accomplish transportation.

Its also about speed and convenience. We've made train travel painfully obtuse. Other countries don't do it this way, so people spend far less time in the air.