I've always wished for a network of rail flatbed cars that make high-speed runs from state to state.
You could drive onto one of the flatbeds (from a ramp) in your car, then ride the train from center-of-a-state to center-of-the-next-state. After a brief layover (for cars to get off and new ones to get on) the train would depart for the next state.
It just seems like a decent way to enable medium-speed, long-distance travel. And wherver you went, you'd have your car when you arrived.
In Europe, some of the nightjet lines take cars as well.
I think this is a great solution. If the travelling takes too long, just do it overnight. So travel times from 9pm to 9am are actually quite convenient.
Flatbeds would be a bit risky, you can see in both this case and Amtrak's case they are enclosed cars. Apparently the Shuttle is one of the most comfortable rides in the world - as there are three layers of suspension (train carriage, carriage-vehicle mounting, and vehicle suspension) between you and the rails beneath.
There are some small trains in Switzerland that are like this, they are for those places where there are no roads or they are closed for the winter. The train takes on a bunch of cars, leaves and drops them off a half hour later. Super convenient and sometimes even free, and easily the best part of traveling to that particular destination.
Best part of car trains in Switzerland is freeing the mountain passes from cars. Still a lot of cars and motorbikes enjoying hairpin turns and landscapes (Good for them!) but still way less traffic than otherwise.
I rode on a similar service in Switzerland, where it's the only way to get a car across a mountain pass in a couple places. As one would expect from the Swiss it's a model of efficient operation, but even in that context it only seems like it really makes sense when there's a reason, like a giant mountain in the way.
You could drive onto one of the flatbeds (from a ramp) in your car, then ride the train from center-of-a-state to center-of-the-next-state. After a brief layover (for cars to get off and new ones to get on) the train would depart for the next state.
It just seems like a decent way to enable medium-speed, long-distance travel. And wherver you went, you'd have your car when you arrived.