The 6 axle goes through both 6-stud beams.
The axle-connectors do not, so the 24 tooth gears engage the 24 tooth gears
on the wheel axles one at a time depending on the motor direction.

Above and below are the same.

Above and below are the same.

The crown gear on the motor meshes
with the right or left 24 tooth spur gear
depending on the direction of the motor.

The motor sits on the two 4 stud beams.

The RCX holds the motor in.
A more stable structure could be made using plates with ledges,
however the Cities and Transportation set does not have them.