Connect the Tumble Buggy and the Pasco cart together and set the friction adjustment so that the Buggy will spin but not be able to pull the cart.  Then during the demonstration, you can reset the friction so that it is less and the Buggy (train) is able to move.  

Place the block under one end of the track and adjust the incline so that the system just moves down the incline as the Buggy is trying to pull it up the incline.  Take a weight off of the cart and place it across the front of the Buggy and observe that now the Buggy and cart system is able to move up the incline.