Thanks for your input everyone.
Came across this interesting youtube video (below) about the DCS/DCC switch, especially the part about adding the switch to Railking engines. Near end of video fellow mentions having sent a Railking engine to "some talented and kind repair person in southeast Pennsylvania" to add the switch. I wonder if that repair guy was electronic guru, Gunrunnerjohn.
TO VIEW VIDEO SEARCH YOUTUBE-
820 MTH Trains, The Elusive DCC/DCS Switch on the MTH Proto 3, O-Gauge 3-Rail Locomotives