Comment by under-Peter

I agree that the problem here has no easy, general and automatic gps-based solution but I think it's helpful to keep the actual likely situations in mind. Usually we have the most trains/densest timetables close to major transport hubs where speeds are low anyway and driving 'on sight' is a good compromise of safety and keeping things working. Where speed really matters - outside of major hubs, straight ruralish routes - we simply don't usually have great density of trains AND tracks. At least from my travels in western Europe I don't think I've ever seen more than two tracks in the same direction on high-speed-segments. So if we keep that in mind and don't expect a general and automatic solution, a simple, gps-based fallback-system that requires all trains that are within a radius of X of each other (either in time or space) MUST coordinate, else they both shutdown/move on sight, could work reasonably well. In reality and for the involved parties (the trains that are close to each other), it's a very human-scale problem.