Wouldn't the locomotion of the train be considered an external force?
As soon as the object is released, the golf ball will begin horizontal deceleration, losing momentum as it is no longer being directly acted upon by the power of the train. (apart from the air-which is also being acted upon by the locomotion of the train)
Ball hits behind the bulls eye.
I'll also argue that the ball will bounce and roll towards the rear of the train, eventually coming to rest at the back wall.
Unless of course our happy experiment is abruptly halted due to the failure of our myopic engineers to account for that tunnel...