Although I've been an O-gauger for years, I'm trying to help my son with his HO layout. We have trouble understanding the hookup from the tender wipers to the loco. Right now, the loco moves okay on its own. as soon as the tender is on the rails, the loco won't work. I've tried different wiring solutions with no luck. Could someone pls. explain how the tender works and why it has a resistor or capacitor inside it? Is it only for the light hookup to the loco? I notice the tender also has a light on rear. No lights work so far. Thanks!