Forcing trains to slow for turns is not a complex engineering problem, IMO. Here's my safer, low-tech solution that wouldn't cost billions of dollars and wouldn't be vulnerable to hackers. Before the turn, pound a "stake" into the ground. Then add a simple lever under trains that forces them to slow down. When the lever hits the stake, the train is forced to slow. No human intervention required. If you need different speeds for different turns, just position stakes further left or right. 20mph stake slows the train to 20mph, easy. (Extrude the metal enough that it wouldn't be easy to tamper with.) Seems like this billion-dollar wireless solution would be vulnerable to hackers. Personally, I'd feel safer with a mechanical solution.