The system I heard Europe uses, and Amtrak is supposed to use on the NEC, plus something like whatever coal/power company ran on their automated train is based on electronic tags in the middle of the track - think a system like proven using the EDI tags on the sides of cars.

Now consider GPS - what if there is a massive solar flare or war that knocks out the GPS satellites? The GPS based PTC system becomes worthless.

I really love electronics, but only when they consistently work as intended, aren't obsolete within a year, and actually do what you need them to (and don't try to "help" too much). Can you imagine just trying to update software changes on all the PTC locations and locomotives? And as complicated as the software is going to be including the stop distance algorithms, sooner or later that little .1% error in the software or operating system will cause a really big boo-boo.