a person sitting on a moving train and facing the engine and tosses a coin which falls behind him.find reason

If you throw the coin up when you are accelerating, for the time the coin is in the air the only force pushing on the coin is the air and that's not dense enough to affect its path. So while you're going forward the coin doesn't keep up and will fall behind you.

This happen due to Inertia of motion. If the train is accelerated horizontally the velocity of the train more than that of coin when we tossed the coin upward it remains constant throughout its flight by the time of can return it falls behind the passenger.