Method to obtain unidirectional polarity in train lighting: Dynamo has rocker arm mounted on the shaft, friction tight. When the direction of rotation of the shaft is clockwise, terminal X (positive) touches P1 while terminal Y(negative) touches N. Thus, giving the output polarity as shown in figure. When the direction of rotation is anticlockwise, X (negative) terminal of the rocker arm will now touch to N1 brush and Y(positive) terminal of the rocker arm will touch to P . Thus, the output polarities will be obtained as shown in figure. Thus, unidirectional polarity is obtained.