I've tracked buses to the less than a meter accuracy using three satellites, with ease.

Also, most of these systems can be designed to check against a gis data base of the railroad, that data base can be built into the system, this allows you to compare the expected position, an inertial positon, and the position with the satellite, looking for exceptions.

Building the data bases of the railroad itself make it much easier to use the gps based systems. It's been done on interstate highways and freeways, including gis mapping every lane, ramps, speeds, including restriction in curves. Signal positions and speed limits could be in the data base, things like track warrants, temporary restrictions, and the like can be updated into such data bases.

A data base allows such systems to compare the actions, and the locations, and determine if the proper speed, slow order or signal indication are being complied with appropriately, if not, make appropriate warnings, if no response, initiate action and stop the train. A variety of methods of communications options exist for transferring data on signal aspect. With the rigtht protocols, you can even detect trains around you, there heading, speed, location, rate of approach, which track they are on.

This technology all exists today. You may be using it on a train simulator next to your laptop as you are reading this message. Like the data bases of train simulators with various runs programmed into them (Cajon, TGV, etc.), only comparing what is supposed to be there to the real world, and making sure that nothing is out of the sync. Kind of like your brain is supposed to do when we are driving a car or a train, but sometimes doesn't.