Honestly, I tend to agree with the railroads: The technology isn't there (British Rail had AWS starting in the 50's/60's, and Germany has had a similar system for several decades, but they're both on NATIONAL systems, and didn't need to communicate between numerous different railroad companies) and honestly, it's a magical miracle cure for railway safety. That said, however, if we want to go with High-Speed Rail (or just speed up our existing Amtrak trains) we need to have some sort of PTC in place, because at 90-110mph human response time to any problem is dramatically shorter; not so much that the human is slowing down, but that they have less time to act on any danger they see.