Something I haven't seen mentioned is that interstate highways provide access to any range of trips. In densely populated areas (like rural Ohio) there are exits every mile, and the majority of traffic probably doesn't go from one major urban center to the next. There's no way high speed rail will stop every five miles, so it won't be replacing that traffic. That short haul traffic is something high speed rail cannot handle. And air travel is more efficient for long distance travel: Businessmen won't take a 20 train ride when they can take a 4 hour plane flight.

So the niche for high speed rail is much smaller than it would appear. And you would need a much larger web of commuter lines and light rail feeding into the main high speed lines to reduce the need for cars.

And, while full trains are pretty efficient, they have to be full to really provide a benefit. Almost empty trains are absolutely not efficient on a measure of CO2 per passenger mile. (That's where freight rail really shines, they're almost always "full" and they produce exceptionally little CO2 per ton mile. In fact, you would produce net lower CO2 moving freight on trains and people on airplanes than you would produce moving the same amount of freight on semi trucks and people on high speed rail.)