Remove the black wires from the front truck off of the circuit board.  Extend them to the back (cab end) of  the circuit board and solder them as shown in this photo.  The Red and black wires from the decoder are solder to the board as shown to get track power. Important note: the leads coming up from the rear truck are crossed left to right. Unsolder them and  re solder them such that the lead from the engineers side solders to the circuit board on the engineers side and the lead from the fireman's side is on the fireman's side.  See second photo at bottom below.

This is cab end view.  Notice the truck leads are attached right side truck to  right side circuit board,  left side truck to left side circuit board.  They are no longer crossed as they were originally.