A high speed train carrying automobiles between cities would alleviate congestion and reduce greenhouse gasses. The train would be built coincident with existing Interstate highways, and would be supported in the medians so no land need be purchased for a 2-way system. The tracks would be level or raised as appropriate, and elevated or sunken at overpasses.
Angled drive-on rail cars would securely hold 20-30 automobiles per car, passengers would remain in their vehicles or in a lounge level, and drive ahead off the rail car at the destination. Like entering and exiting a Toll Plaza, the cars will merge onto rail cars at the beginning and destination.
To be practical, speeds would be in excess of 150mph non-stop between major cities. Other intermediate trips would be on ground.