It would be useful if you could drive your car onto the rail and it carried you and your car at 300 mph.
IIRC the Auto Train is the only long-distance route Amtrak runs that is net profitable https://en.wikipedia.org/wiki/Auto_Train It's possible other corridors could be identified that could run something like that, though it probably needs to be about 800-900 miles to make it "worth" it time wise - shorter and it's simpler to just drive.
The auto train to Florida is sort of a special case. That’s not people traveling for a long weekend or even a week. It’s historically been people going south for the winter. Not sure it’s profitable though don’t see recent numbers.