Railway is a different problem space than road vehicles. Trains run on fixed tracks, has long braking distance, are much heavier, and accidents can be much more devastating. While self-driving cars may use cameras, lidar and radar for detection and brake on sight, trains need to know exactly how far they are allowed to go and what speed restrictions are ahead in order to brake in time.

Traditional lineside signals and signs do give enough information to drive a train by a computer, but the rail industry has already chosen decades ago to integrate signalling and automation with the infrastructure, which is a completely different approach than today's self-driving cars. As a result, heavy rail and metro trains will not be "self-driving" in the same way as self-driving cars.

Now, if you say trams that run on streets, then possibly. Trams are often driven by sight and has stronger braking so it is possible to apply the same principles from self-driving cars. In fact, Siemens has been developing and testing one such tram. [1] Though I think the problem space is very similar to self-driving cars. There is no steering, but it still has to handle pedestrians and other road traffic.

[1]: https://www.mobility.siemens.com/global/en/portfolio/rail/ro...