Question!

Larry. The only expierence I had was two lifelike SD 7's I got off E Bay. They had the decoder installed by a train shop and when I received them I tried them on a test track. They would not run. I had to take the decoders out and rewire them before they ran properly.

I have seen other post where it was mentioned that some engines that were decoder equiped or had the plug in for decoders would run both ways but I have not seen any myself.
 
Larry, No easy answer here. Digitrax decoders will run on Dc, unless someone has programmed them not to. These are the only ones I have first hand experience with. I have several Soundtrax decoders, but never tried them on DC, I have read that they do not work on DC. Likewise, the one NCE decoder I have has never been tried with DC. Perhaps others can say for sure.
 
Larry,
A Digitrax decoder comes set from the factory to change automatically from DDC to DC. You can change CV29 to turn that function on or off. I don't know about the other manufacturer's decoders.
Marty