Driverless trains are not a problem to begin with. If you have a train and it carries hundreds of people [or trainload of cargo], it is economically irrelevant whether it has a human operator or not. So it is safer to have a human there just in case.

The tech for automated road trains is vastly closer than where self-driving cars are. If you feel this way about trains, then you should feel the same way about highway-specific automobile autonomy.

We also need to focus on the actual problem we're faced with. Cities are hitting limits to growth due to congestion and we can't build more roads. The state of tech right now is capable enough to solve that problem by increasing the throughput of the major arteries of LA / DC / etc.

The ECA has recently made an assesment of high speed rail and they found out things are not as rosy as they seem:

https://www.eca.europa.eu/en/Pages/DocItem.aspx?did=46398

That link requires HTTP basic auth.

But my understanding is that at least as far things like subways go, driverless trains aren't a difficult problem.

Vancouver, BC has been operating a driverless train system for its rapid transit since 1986.

I think the competition for cars are not high speed lines, but normal, everyday commuter trains.