With electronic controllers the issue is which way electricity will go through the controller. Solid state devices only conduct in one direction. With some electronic controllers you must have a brake circuit or you will only have full speed, other controllers do not have to be connected to the brake circuit, but both types are polarity sensitive. Several makers have dual polarity models. With positive polarity tracks the brakes are connected to the negative side of the power supply and to the left hand track rail looking in the direction of travel. The wiper is connected to the right hand track rail and the full power connection goes to the plus side of the power supply. With negative polarity tracks the brakes are connected to the positive side of the power supply and also to the right hand track rail. The wiper is connected to the left hand track rail and the full power connection goes to the negative side of the power supply. 1/32nd plastic set type tracks all use negative polarity.