Train drivers do more than just drive a train. They also have to deal with dropping off and picking up cars from the consist. They also need to monitor track conditions that aren't necessarily easy to pick up from track circuits, such as deep snow (https://www.youtube.com/watch?v=Yja2VmZOfdA shows you how some trains deal with snow).
In general, the costs of upgrading track to be able to support autonomous driving aren't worth the savings you get from eliminating driver. The main counterexception to this is Western Australia, where everything of value is 1,000 miles away from where anybody lives (and is transported by trains to the coasts for export), and so railroads face tremendous costs actually getting their drivers to their trains. Unsurprisingly, the mining companies in Australia who own/run their own railroads have invested heavily in autonomous trains.