I think we need a bit more information, to be honest. Is this engine 2 rail O (if you look at the trucks, does it have the roller for the third rail?). If not, this is a 2 rail O and as someone else said, reverse the polarity of the power coming in (swap the neutral and hot lead) and the direction will change (basically the same thing as an HO scale engine, where on a simple power pack there is a direction switch that reverses the polarity).
Three rail normally uses a reversing unit, on older engines it was a mechanical unit called an e-unit, that used a solenoid and drum to reverse polarity, later engines use solid state units to do the same thing. In theory if you put dc right to the motor and reverse the polarity of the wires as far as I know it should reverse it as well, the e-unit was allowing a remote changing of direction,at the motor it is reversing polarity the same way.
