The bogie is held in place with a pin. This protrudes slightly at either side, and keeps the bogie locked in place. The protruding end also prevents the bogie from turning too far.
In addition, the pin acts as the axle for a plastic gear which transmits power from the worm drive to the bogie wheels. Push it in from one side with a small screwdriver, and then pull from the other side. It isn't a tight fit.

The engine body can be lifted off the bogie.

And the plastic gear can be removed from the top before it drops onto the floor.

The Gears can be accessed from underneath by removing a single screw and a sturdy metal base plate. Note the brass bushes on the axles.

The wheels and the gears lift out with tweezers. Two sets of wheels, two brass cogs.

A closer look at the pickups and the contact strip. Note the amount of fluff and hair in there already.

And again...

.... and again.