 So in Alice's frame of reference, Alice has zero velocity. She was not moving, and this is the rule. Even if I'm sprinting down the street at breakneck speed, in my frame of reference, I have zero velocity. I'm not moving at all. And this is because my reference frame, my coordinates, are sprinting down along with me, and so that I'm always at the origin. And if I'm always at the origin, then I have zero velocity. So what about the train? Well, the train is also moving along with Alice. Each point on the train stays at the same coordinates relative to Alice. So the train also has zero velocity in Alice's reference frame. So now let's ask what happens when the ball reaches us. How fast does the ball collide with Alice's hands in Alice's reference frame?