Car-train!

Some sort of railroad car. Car-trains are a possible key advance in transportation technology. They have the potential to alleviate congestion by increasing throughput (number of vehicles per lane per hour) by a factor of at least two. Vehicles that are physically coupled or able to follow with a gap of less than 1 foot can reduce aerodynamic drag more than 50%. However, significant development issues regarding the controllability, safety and new driver behavior remain to be resolved...