If you have a positive side, then the barrel loader won't work... It must have AC (alternating current). Alternating current is required to produce the vibration that moves the barrels. So the first thing you need to do is verify fhat the MTH Z-500 Transformer is indeed AC. The second issue is to figure out what you will do with a transformer that does not have accessory output terminals. 18 volts is way too much voltage for the barrel loader. As far as the question about which terminal has the switch attached to it.... It does not matter. Look at the Lionel drawing, in the top view the switch is attached to the terminal closest to the giant man. In the bottom view, the switch is attached to the terminal farthest away from the giant man.