When you pack people into a train, you fit about 1 person per m². Pack people into a single-occupancy cars, you get about 1 person per 10 m², or 0.1 person per m². That's a 10× capacity loss that has to be made up somehow if you want to move people as efficiently as possible.
Autonomous cars don't solve the problem of inefficient use of land for transportation (actually, they probably make it worse since there will be a decent fraction of cars moving absolutely no one). Space is at an absolute premium in cities, and people really underestimate just how much space cars take up, particularly since that space is incredibly subsidized.
A purely autonomous road system would make much better use of space than the current system. For instance, you could discard with the notion of fixed lanes on a motorway as the system could nearly instantly allocate lanes on the basis of demand. You'd also be able to do away with curb side parking in urban centres, which would free up more land for commuting.
A train is obviously more still more efficient between fixed points, but self-driving cars are far more economical for less-frequented routes and more convenient for last-mile transit than buses.
Autonomous cars don't solve the problem of inefficient use of land for transportation (actually, they probably make it worse since there will be a decent fraction of cars moving absolutely no one). Space is at an absolute premium in cities, and people really underestimate just how much space cars take up, particularly since that space is incredibly subsidized.