I'm assuming it has inertia, so no, it would continue to move forward and just fall to the the ground of the train. But has anyone really performed this experiment in a vacuum? In the absence of vacuum, for example, if the train is filled with air, wouldn't the air be the one that's pushing the ball forward? That would explain why the speed of light is always constant, because the speed of light is not affected by any medium, meaning it cannot be pushed by air for example.

I hope you believe in the saying: "there is no silly question!" :)