I'd vote for the other way around. Combine the flexibility of the car with the freedom of riding a train. Let people drive manually to predetermined locations,

I'd vote for the other way around. Combine the flexibility of the car with the freedom of riding a train. Let people drive manually to predetermined locations, let's name them a "station". Where the "AI" takes over and lines up cars on predetermined routes, let's name them "tracks". The AI will control the car on the "track" up until the nearest "station" to the destination. From there the driver will manually continue to her destination.