The best explanation of what happens to Amy is:
While Amy is walking, her body is in motion. When the train stops, because she's not buckled in, Amy continues to move forward due to inertia.
This explanation accurately describes the principle of inertia, which states that an object in motion will continue to move in the absence of an external force acting upon it. When the train stops suddenly, Amy's body tends to keep moving forward because there is no force applied to stop her immediately.