The tender also picks up power through the wheels.
With the old Playmobil locos, LGB only provided the tracks and the motor is made by Playmobil and takes a maximum of 16v, so ideally you should try and buy a Playmobil transformer/controller (a single unit which does both jobs). These often appear on eBay.
LGB transformers and controllers can deliver up to 22 volts, and could burn your motor out.
You can also use the Playmobil trucks from your battery powered Playmobil train.