Putting your car on a train and riding with it removes a long day of driving. The service exists on very few corridors, and the reasons are operational rather than a lack of interest.
Loading needs dedicated terminal equipment
Vehicles are driven up ramps into enclosed auto racks, which requires a terminal with hard standing, marshaling lanes and ramp structures matched to the rack design.
That equipment cannot be improvised at an ordinary station platform. Each end of the route needs a purpose-built facility, and the capital cost only makes sense at high volume.
Loading also takes time. Several hundred vehicles have to be driven aboard, chocked and secured, which is why check-in closes hours before departure.
Intermediate stops break the model
Because vehicles are loaded in sequence, a car stowed early is blocked in by everything loaded after it. Unloading at an intermediate point would mean shuffling the whole rack.
The practical answer is to run non-stop between two terminals. That is why these services are point to point rather than a line with stations along it.
Non-stop running has a second benefit. A long, heavy train with no station work can hold a schedule that a stopping train on the same track could not.
The demand has to be directional and balanced
The service needs enough people making the same long drive between the same two regions, and it needs roughly equal numbers going each way to avoid deadheading empty racks.
Seasonal migration patterns produce exactly that balance in a few corridors, where one population regularly relocates for part of the year and returns.
Most long American drives do not have that shape. Traffic is dispersed across many origins and destinations, which cannot be served by a single pair of terminals.
Train length creates its own constraints
A consist carrying passenger cars, sleepers, dining service and a long string of auto racks becomes very long, which limits which sidings it can fit into when meeting another train.
It is also heavy, so acceleration is slow and the schedule has little recovery margin built in. A single delay tends to persist to the terminal.
Handling that length requires route selection with adequate siding capacity, narrowing the candidate corridors further.
What it means for planning a road trip
Where such a service exists, it converts a long interstate haul into an overnight, and the trip begins with a car that has not accumulated the mileage or the driver fatigue.
The trade is rigidity. There is one departure, one arrival, and a vehicle release order you do not control, so priority unloading is usually sold as an extra.
The economics work best for travelers who would otherwise spend two days and a hotel night covering the same distance, and much less well for anyone wanting to stop along the way.