Some are. Retrofitting infrastructure necessary to facilitate reliable, safe autonomy in all conditions is expensive and building systems to meet expectations in all conditions is also.<p>Many railways grapple with issues of reliability and accuracy despite being partially or fully automatic.<p>With cars, you have the easy out that the responsibility always rests on the driver. If you mess up, it's their fault; their hands should always have been on the wheel. If you break it, one person is late for work. If you break a rail network, everyone is late for work. Different risk profiles.<p>Being able to ship code of dubious quality and iterate on it is preferred for companies rather than engaging in a massive contract to deliver a perfectly working rail inventory and routing management system.
There's no practical difference between 'tram", "train" and "light rail".<p>There are lots of autonomous trains all over the world. Many airports use them. Many city subways use them.<p>You probably haven't been looking hard enough. They are there. The first time you ride in one can be a little unnerving when you can look out through the front window and there's no driver blocking your view.
Here the current underground trains are <i>almost</i> fully autonomous. They still carry a person in the cabin "just in case" and mostly because of union pressure. There's also a plan where new trains to be delivered in 2025 will be fully autonomous and have no driver at all.<p>AFAIK the situation is similar in some other European cities.