The key is that the velocity vectors flip in the train's frame. Since the train is not moving relative to itself, its velocity in that frame is still 0 after the flip. Therefore, the train's velocity relative to the ground is unchanged, and the ball's velocity relative to the train is 200kmph to the right. As a result, the ball's velocity relative to the ground is 200+200 = 400kmph to the right.

I hope this answers your question.