Comment by cyberrock

This isn't as simple as a Disney monorail. There can be many tracks, multiple routes can share some segments of track, trains can go from one track to another, tracks can be stacked on top of each other on some bridges and go inside tunnels so GPS/GNSS is insufficient, the train in front of you can stop for emergency, etc. It's like the aeronautical collision issue, but the tracks are in charge of determining and transmitting position.