I'm fairly certain this is a bug, but when you place down any minecart with an object inside (furance, chest, tnt) the direction of what that block changes when on the rails. So when I face west and place a furnace minecart west, it faces east. And when I then turn east, it only faces east. So the minecart is only facing certain directions on the rails and when on a turned it turns the opposite direction for a split second and then reverts back.