Appreciate all the quick responses.  However, I really wasn't thinking when I posed this question.
Reversing the pos & neg connections to the track got it to work... as long as all the trains are going that one direction.  A DCC loco knows the difference between forward & reverse, but a DC loco doesn't.  It only knows which direction the current is sending it.  So for a (DC) train going the opposite way, forward on the track will be reverse on the throttle. 

I have an old Bachmann F7 (DC) on a DCC layout.  It runs the opposite direction than my DCC engines.  How difficult is it to rewire the diesel so that forward on the throttle is the same direction as the DCC locos & what wires need to be changed?  Thanks.

I was expecting shipping info & prepayment options.  Did not realize that the RO# was all I needed to get the ball (& the loco) rolling.  Thanks for the quick response.

I have a Spectrum 2-8-0 (HO #83601) that needs repair.  (Train was left unattended on a Christmas layout & got hung up on a plastic tree that fell over the track.  Believe the gearing is damaged.  Also messed up the side rods/linkage.)  I've sent two service request to Bachmann & have gotten no response (other than providing a repair order number).  Is there anyplace (else) that I can get this engine repaired (out of warranty)?