[botten]

There are fully automated trains. However full automation requires not just a capable "train cockpit" but also a capable signal system (with supporting auxiliary systems). Introducing the latter is, as far as I can tell, the hard/expensive part.

As a note, not all trains have fixed speeds or fixed stops. As an example some signal systems may allow a train to enter a section where another train is already present if the driver drives slow enough to be able to stop before colliding.