As others have pointed out, cars can go directly to your end destination, which trains cannot do.
But aside from that, I suspect we'll see at least a doubling of car travel speeds in the next few decades.
Speed limits are an artificial constraint in the name of fuel efficiency and safety. With automated cars, the safety limit is at a much higher speed, and even fuel efficiency can be improved with computer controlled 'trains' of cars drafting and reducing wind resistance overall (made possible by automation making the idea safe).
Still a long way from bullet trains and aircraft, but much better than car speeds today.
There are trains with rubber wheels...anything that has to deal with grades. They are still tracked, but the advantages are a bit less. Trains support more density, I think you'll see automated cars and trains support each other.
If we could feasibly build roads that better dealt with wear, we would do so, instead of spending hundreds of billions of dollars each year repaving them. Highways today aren't dirt wagon roads, but neither were the highways Romans were constructing two thousand years ago. Meat space technology advances slowly.
The "train of cars" proposal is just absurdly inefficient. Carting on or two individuals in one-ton steel cages on highly inefficient rubber tires, versus a train running on a steel track. All the inefficiencies in play go up dramatically with velocity: energy lost due to deformation of the tire, energy lost due to friction, wear on the road, etc.
You seem to be wedded to the idea that one thing will change and the rest of the system will remain the same, thereby making the original change impossible.
That's just not how things work.
Roads are made the way they are as balance of various tradeoffs. When one part of that equation changes, other parts will change as well to adjust to the new balance.
For example, if all cars were fully automated, safety margins would allow roads to be notably narrower, requiring less construction and maintenance costs all around. In addition, increased throughput could have much the same effect- no need for so many lanes.
And of course cars themselves are the same way- a system of tradeoffs balanced to optimize for current constraints. Change one of the constraints and the system changes. Cars won't be "one-ton steel cages" if they don't need to be. And lighter cars will reduce road wear, as vehicle weight is a primary factor. Tires formulated for optimal performance at current speeds (and weights for that matter) will be reformulated.
Systems are complex, and changing one variable means the rest will adjust.
And you seem to be wedded to the idea that change can happen regardless of physics. Automated cars isn't going to eliminate the need for a certain level of crash protection, and reformulating tires isn't going to change the basic physics of energy loss due to deformation versus the need to have deformable tires to get adequate traction.
There is a reason advances in transportation (or energy) technology are so slow and expensive. The physics really are not in your favor.
Current systems are optimized for current constraints. When a constraint changes, the entire system changes.
You can keep saying "Physics! Physics!", but the entire point is that once you change the one input in the physics equations, you can change all the others to keep the balance.
A change in one constraint might allow you to move to a different point in the design space, but it doesn't change the physics by which the system must abide.
More concretely, replacing a human driver with an automated one isn't going to change all the physical constraints that make it inefficient to have cars that cruise at 100+ mph: http://energy-ecology.blogspot.com/2010/05/optimal-vehicle-s.... Whether you're in a Honda Civic or a Ford Explorer, doubling speed from 90 kph to 180 kph triples fuel consumption.
Even if self-driving cars can be a little lighter, that's not going to change the shape of the curve.
But aside from that, I suspect we'll see at least a doubling of car travel speeds in the next few decades.
Speed limits are an artificial constraint in the name of fuel efficiency and safety. With automated cars, the safety limit is at a much higher speed, and even fuel efficiency can be improved with computer controlled 'trains' of cars drafting and reducing wind resistance overall (made possible by automation making the idea safe).
Still a long way from bullet trains and aircraft, but much better than car speeds today.