A passenger in a moving train tosses a coin upward which falls behind him. It implies that the motion of the train is

When a passenger tosses a coin upward in a moving train, the coin initially possesses the same horizontal velocity as the train due to inertia. If the train's motion is uniform, the coin will fall back into the passenger's hand because both maintain the same horizontal speed. However, if the coin falls behind the passenger, it implies the train has accelerated forward while the coin was in the air. During the flight time, the train's velocity increases, but the coin continues to move horizontally at its original, lower velocity because no horizontal force acts upon it once released. Consequently, the train (and the passenger) moves a greater horizontal distance than the coin, causing the coin to land behind the passenger. Conversely, if the train were retarded (decelerating), the coin would fall ahead of the passenger.