I have what I think is same problem/question. I know it has to do with short engines passing over the "frog" in the switch.  I am getting back into N after being gone over 30 years and catching up with the advances in technology.  I have all EZ track and about 30 switches, both plastic and metal frogs.  I want to continue manual operation of all these using the black direction switch provided by Bachmann.  Is there an easy, cheap device to electrify the frog and automatically change its polarity for DCC powered track?