You might say that the ball would fall behind the person who throws it because he would have moved forward with the moving train. But in fact this is not correct.
You can perform a simple experiment to answer this question. You would be surprised to find that the ball lands right in your hand when thrown upward inside the moving train. Do you know why it happens so?
In a moving train everything inside the train also moves with the speed of the train, for example, the fans, passengers, you and the ball in your hand. When you throw up the ball, a part of the speed of the train is imparted to it. It acquires a vertical motion in addition to its horizontal motion. The passengers in the train cannot see its horizontal motion but only its upward and downward movements.
Imagine a man outside the train, who is watching your experiment. As we have said the ball possesses both vertical and horizontal motions, both these motions combined together make the ball travel along a parabolic path. The observer outside the train will see the ball moving in a parabolic path but a passenger in the train will see only the up and down motions of the ball.
Now the question arises whether the ball follows the parabolic path or just moves up and down? Out of these two which one is right? In fact, all motion is relative to the observer. There is nothing like absolute motion and hence the motion of the ball is different for the two observers.