I get that it's moving fast, but what's the specific threat model? Terrorists hijack the train and somehow drive it off the tracks? Presumably it's easy enough to secure the "cockpit". They could blow it up, but is that dramatically worse than blowing up a train moving 50 mph (genuine question, I'm not sure about the physics)? Especially as these trains don't carry fuel, I can't imagine a tragedy on the scale of 9/11.