Thanks for the reply DrClaude. Right after my post I read an excerpt from Galileo Galilei's book and I understood how it works (in Galileo's case he was testing moving objects within a ship's hull).
Can someone answer this question? If I throw a ball forward and another one backward (opposite to the train's movement) in a moving train, do both balls land at an equal distance from the thrower? I suspect throwing a ball forward is the same as throwing it in a motionless train but what about...