I was recently fiddling with one of these myself.Service sheet - https://www.lendonsmodelshop.co.uk/pdf/Airfix Locomotive Service Sheets/Prairie Tank.pdf
The driving wheels are plastic centered so current pick-up is by wipers on the back of them which are linked toether on the inside the plastic baseplate. The service sheet however only shows one wire from that baseplate so really you need a multimeter or something else to check the circuits; looking at it your photos it would seem that their are two wires from the baseplate to the motor but you need to find out if the metal chassis is live to the pick ups and or motor,
There are indeed pickups on both sides, however, inside the keeper plate, one side of the pickups is connected to the wire you mention. The other side has a tab which rests against the chassis block, meaning that the chassis block is live.
This may be an issue with the motor in that it too may not be insulated from the chassis, meaning that you can't isolate the motor for a decoder to sit between it and the pickups.
I will double check.
In the end, I actually replaced the chassis on mine with the Hornby version of the same thing (the version prior to the latest 'hi spec' version).