After installing a decoder in my mountain, I started having issues with it. The back light would work and it would back up okay, but as soon as I hit forward, it would take off at full speed and the only way to stop it was to turn the power off. I tried it again and the same thing happened. I switched decoders and the problem went away. I suspect a bad decoder. I have installed about a dozen decoders and thats the first time that has happened. Strange!
This page might help, scroll down to "Speed Control".
http://www.dccwiki.com/DCC_Power#Polarity_doesn.27t_control_direction
As I stated the problem went away as soon as I installed a different decoder. So the decoder was bad not my installation techniques, although they could improve greatly.
The page just tells what might have been the problem with the bad decoder, as a matter of interest only.
Sorry, didn't catch it. :-[