And yet, I am positive it is not a better way to control trains. The fact is, GPS is NOT the end all, be all when it comes to position, navigation and timing. It is a great technology, but it is not what the general public thinks it is. The fact is, the military - who developed the technology - is constantly looking at ways to supplement it and in its control systems, only uses it as a secondary control source. Additionally, there are limitations and shortcomings which are inherent in using GPS as a PNT method. For example, when a train is moving at 79 mph, it is only accurate to around 12-15 ft - accurate enough for single track operation, but not enough for multi-track operation. In order to make it work, you must add infrastructure and implement a dGPS system, meaning there are towers providing known position data to increase accuracy at speed (and these towers ideally must be placed sufficiently away from the right of way to provide the necessary increase in resolution needed). Next, it introduces a security issue, given that train movements are critical to the national economy and passenger traffic is placed at risk. I have seen little in discussions about PTC which address the security implications of using a radio based control system (including GPS). The US military spends large amounts of money to counteract these security threats, yet the PTC implementations I've seen do not seem to adequately consider them. If I were an executive for a railroad, I would balk at the cost of PTC and the necessary security countermeasures for electronic warfare. They are nuanced and mostly classified - something the rail industry clearly does not want to deal with. Additionally, the improvements in GPS-based train control systems only provide a marginal improvement in performance over traditional track-circuit based systems. There are other issues as well. GPS is great - I use it personally all the time and deal with it professionally always. So, after having worked closely with navigational systems since grad school, I am definitely not convinced it is better for rail applications. Sure, its cool and sexy, but better - I doubt it.