The Drive wheels on the left are all connected electrically to their axles. The axles sit inside a brass bush which is held firmly in place by the baseplate.
All of the left hand side drive wheels are therefore connected electrically to the chassis.
Note that the gears are on the left hand side.
On this model, the valve gear is isolated from the vlave gear on the right hand side. The two halves are held in place by the red plastic mounting black. This is also isolated from the chassis. The left valve gear may be connected electrically to the left hand wheels through the con rod and the bolts in the fron and rear drive wheel. This connection may be intermittent, and is not part of the power supply circuit.