Below is a link to the Spectrum 4-4-0 tender. Lower left you can see the DC adapter. The Bachmann steamers have the same setup.
http://www.bachmanntrains.com/home-usa/dwg/dwgs/H834X-IS001.PDF
I have a new Spectrum 4-6-0 with on board Tsunami sound that came with the adapter in the box just in case a user does not know how to operate a DCC loco on DC or does not like how the loco runs on a DC layout. Bachmann with on board decoders are ready to run on DC as well as DCC.
Your loco should have come with a similar diagram.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
Yes, that plug you see is the "dummy" plug so the engine will run on DC block control. Pull it out of the socket and plug your decoder into its place.
Inside this tender is what looks like an 8-connector plug, but it has no wires leading out of it, only some solder blobs (?) on its top. Is it actually the plug I need to remove and replace with the 8-connector pin that's part of my MRC decoder? I'd hate to yank it off if it isn't.
Neil