Question

A BALL THROWN VERTICALLY UPWARDS BY A PERSON IN A MOVING TRAIN COMES BACK TO HIS HAND. WHY???

Answer 1

Because of relative velocity.

Answer 2

Simply the frame of motion.

Inside the train the man and the ball both are moving with the same acceleration as that of train.
You can see it practically by staying near(more than 100 meter) a train.You will see the ball and man both are moving but the man inside the train will see both are at the same position.

This is relative velocity concept.Frame of reference is the trick here.

Thanks.
Tripathy.