The requirements of PTC go beyond anything a car's navigation system has to, most notably maintaining separation between trains: also, unlike a car's navigation

The requirements of PTC go beyond anything a car's navigation system has to, most notably maintaining separation between trains: also, unlike a car's navigation system, because it's a system there to ensure safety rather than provide assistance to the driver, the system has far higher requirements for failing safe. As far as I'm aware, proceeding without movement authority is a far more common cause of rail accidents than overspeed, hence why systems have traditionally been mostly concerned with separation and movement authority rather than speed. Really the biggest flaw of the US's push for PTC is reinventing the wheel: ETCS[0] has been standardising this all over Europe (with parts, such as GSM-R, being adopted in large parts of the world!), and the goals for PTC largely match up with ETCS Level 2, as far as I'm aware. There are a number of off-the-shelf systems available, leaving the largest issue as GSM-R frequency acquisition. And, I suppose, the political issue of using a non-American system. ETCS Level 3 essentially allows pure-radio positioning, but there's currently no planned deployments because nobody yet has constructed a system that meets the required "safety integrity level". Note that one of the issues here is you need devices at both the lead and rear vehicles to ensure train integrity, whereas currently typically one only has such equipment in the driving vehicle(s). There's hope to use the EGNOS satellite augmentation system to get better accuracy guarantees than GPS/GLONASS/Galileo provide, but it's all still experimental, and the only experimental project there failed. [0]: https://en.wikipedia.org/wiki/European_Train_Control_System https://en.wikipedia.org/wiki/European_Train_Control_System (an article which I will openly say defines far too little jargon it uses!)