Train-based transport works best if large groups of people all want to go to the same places, and don't mind hanging around for long periods of time waiting. But the overall philosophy (force people to go out of their way so that you can bulk them up into large groups, then use economies of scale to
take them all together less frequently) is often seriously sub-optimal.
There's lots of inefficiency having to stop at stations you don't want to get off at [Non-Stop Metro], having to wait for a train to arrive, having to get off one train in order to transfer to another, having to walk for 20 minutes when you get there because there is no closer station, etc.
Autonomous vehicles are already practical. They can draw on a combination of local robotics (based on their own sensors) and external control (where a traffic management system conducts and choreographs all the various vehicles in its area, allowing it to simultaneously adjust the actions of multiple different vehicles e.g. so that they don't hit each other)
Autonomous vehicles have many of the benefits of a train-based system (you can ride in them and concentrate on doing something else at the same time; they are less susceptible to human error, drunkenness, road-rage, etc)
However, the biggest problem with Auto-autos is how to introduce them into society. Here's where the underground system comes into play ... we have a beautiful system of tunnels, routes, etc which is entirely self-contained and isolated - so its a great initial way of getting people used to the concept.
Initially, you would come into an underground station and climb into a small car (probably a bit like a telecabine - maybe in different sizes with mostly single seaters up to group mini-bus sized things). You would enter your intended destination and your 'car' would zip off via the best route, merging into the stream of other vehicles where necessary and e-merging again when a route change is required. The cars would probably even use the existing electrical power supply. The control system would optimise everything, with much greater flexibility because these cars don't need to be on rails. Also, because they are little, they can accelerate much more quickly - and given the combination of central control and robotic autonomy, higher speed manoeuvering can be much safer
Big wins: no more strap hanging with people buried in your armpits or vice versa. No more being forced to share your space with a bunch of zombies who look as if they're eyeing up your brain as a nice little breakfast snack. No more getting frustrated because the person whose book you are reading over their shoulder doesn't turn the pages quickly enough. No more 'guess the song' games to play with tinny headphone pollution.
Later, you might buy a car for normal everyday road use, but one which comes with an autonomous control capability. You drive up outside the station, the control system negotiates handover and gets your destination, and the car then drives itself through the station entrance, down the connecting tunnels etc until it pops out again where you wanted to get to, and hands back control to you so you can continue your journey.
As other road networks come online you would find the car taking over for you as you come onto the motorway, or as you hit a city centre one way system.