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Test Forward and Reverse Operations
Next you need to test your train set's forward and reverse operations.
All power packs (unless they are very old) have a direction switch for selecting forward and reverse operation. Often there is a "brake" position in between the forward and reverse settings. Instead of a switch, some older power packs have a "zero" position in the middle of the throttle, so the throttle itself does the reversing depending on the direction you turn it past zero.
A two-rail DC locomotive reverses direction when the polarity of the DC voltage applied to the rails is reversed. This causes the motor to reverse its direction of rotation. The brake setting, if there is one, turns off all power to the rails. The photo shows a power pack with the throttle at 50 percent and the selector in the "Reverse" position.