gp15-1

Is it new? Is it DCC or DC ready? What is it doing?

Because it is new, you have to decide if you want to return it or try to fix it your self.
A simple check would be to put it on it back with the wheels up. Then with the track power leads touch a wheel on both sides of the rear truck. Have the power supply set at BB medium voltage. The unit should run with no hesitation and all the wheels turn at even speed. If one of the trucks wheels do not turn, that will act as a brake and the other truck wheels will spin trying to move the unit. This would indicate a drive shaft problem on that side of the motor. Let us know what you see.