You have to be careful when you mix prewar and postwar switchers. For prewar switchers one wire connect the center rail collectors in the loco and the tender. I think this was done to help the switcher get over a 0-72 turnout. On the post war switchers, which all had just four drive wheels, the engine to tender wire is the outside rail connection. This was done to help the short wheel base loco get through 0-22 turnouts. If a prewar switcher is plugged into a post war tender, without some modifications, there is a dead short. If the tender has a second wire, that is there to operate the locos front coupler from the accessory shoe on the tender. If the tender has a third wire, usually green, that is to operate the e-unit from a slug relay in the tender. This was known as “teledyne” reversing. This was not common, found in prewar switchers 232 and 233. I do not recall it this was ever offered in the 20x switchers with the parallel plate motors.