The control of the train itself is easy. Simulations abound for this purpose. The GE's and Siemons of this world wouldn't hesitate to implement them if there weren't other significant rail context specific issues, such as the human environment comment above. The difference is that it's not a human vs human driver problem but a schedule design and human making bad scheduling decisions now that the trains are running late, implementation problem.
Furthermore, train drivers are cheap (compared with other infrastructure investments) and relatively efficient as they can be skilfully taught to drive according to a plan (compared with your fellow road commuters). Without other investments to tell the driver or the computer that they can drive faster/closer to the train in front at most railways are only likely to see efficiency gains (lower power/diesel usage) but struggle to drive those trains to denser schedules. It will happen for non-capacity reasons such as inter network usage, and safety to prevent trains from speeding around curves and falling off.
http://www.computerworld.com/s/article/9134771/The_lost_NASA...