The brown ground connector is used as a return for both the red locomotive / rail power as well as the yellow light / solenoid current.

The red wire from the feeder track's center rail goes to the "B" connection on the trafo and the black or brown wire from the track's base goes to the "O" connection on the trafo. The yellow wire from the turnout goes to the yellow or "L" on the trafo along with any other accessory lights which you may have. The 2 blue wires from the turnout go to a 476/4 Control Plate with pairs of red and green buttons; then connect one of the side connectors on that Control Plate to the black or brown wires going to the "O" on the trafo.

The turnout works from power from the "L" and the circuit is completed when you press a red or green button to make one of the 2 solenoids in the turnout move the points.

The only things which connect to the "B" connection on the trafo are the feeder tracks for the center rail.

Brian