The PATH subway system between NYC and NJ has been boasting on little ads inside the cars that they are implementing communications-based train control and positive train control. How would positive train control work underground, if it's GPS-based?
If I accurately recall, PATH is rolling out Siemens Trainguard MT; a Communication-Based Train Control (CBTC) system which uses trackside (between-the-tracks) beacons and in-car equipment.
Radar is used to figure out the vehicle's distance from the beacons. The system communicates using a wireless-and-wired network to a control center and keeps track of mobile blocks. Here's a brochure [1]. I'm not entirely sure, but I think GPS isn't used in Trainguard -- which makes sense; the beacons know where they are in relation to the track geometry, and locations only need to be resolved in track-space.
