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There are tons of fully automated metros/subways (usually the prerequisites are grade separation and platform screen doors to minimise the risk of accidents). Multiple lines in Paris, the DLR in London, Copenhagen metro, ffs a line on the Sofia (in Bulgaria, the poorest country in the EU) metro.

I think what you're asking for is called "metro". Trams still have to share the infrastructure with cars and pedestrians, meaning all the downsides, with none of the upsides people funding autonomous transit find enticing.

I think that's mostly due to the fact that the small market size makes it not quite worth it. With something like self riving the development costs are huge so you better capture a sizeable market.

These do exists e.g. Nuremberg in Germany has an autonomous subway since 2013.

Lots of cities have these by now. The ones I'm aware of on top of my head are London, Copenhagen and Singapore. I'm sure there are many more.

The difference is probably that they don't have to interact with unknown factors (humans in cars).