Does the system automatically slow down the train, or does it notify the engineer? I would imagine that there are some scenarios where going over the speed limit is the correct choice.
https://en.wikipedia.org/wiki/The_Bullet_Train
https://en.wikipedia.org/wiki/Bullet_Train_Explosion
In the movies terrorists place a bomb on board and the train crew has to maintain a minimum speed or the bomb explodes (this is where that american movie with a bus got the idea). And they have to manipulate the ATC or else it will stop the train when they enter sections of the track with lower minimum speed, or else ATC stops the train and the bomb explodes.