I've found an Atlas RS11 that would fit nicely on the roster. It is advertised as DCC, but the Atlas web site only says it has a DCC 8 pin plug. It is also a road not currently listed on their web site (MP.) Did they have an earlier run of different road names with decoders?
AFAIK, Atlas RS-11's have had three different electronic sets in them. The original "yellow box" RS-11's just had the center light bulb. The first several runs of Atlas Classic RS-11's had a printed circuit board, but no DCC plug (one had to use a board replacement DCC decoder like a Digitrax DH150 or the Atlas Master decoder by Lenz). The later runs of RS-11's have an 8-pin socket.
What you might have is a "yellow box" RS-11 that someone installed a decoder in. The "yellow box" RS-11's are the only ones not listed on the Atlas website, IIRC. All the Atlas Classic ones should be listed.
If you can see the picture of the unit, look at the detail. All the "yellow box" locos had the grabs cast on while all later RS-11's have the wire grabs. Also, look at the trucks. These were cast in a dark gray plastic and were not painted on the old RS-11's.Paul A. Cutler III*******************Weather Or No Go New Haven*******************
If you go to the Atlas website under HO Products and scroll down to RS-11 they have announcement going back to 2000. Unfortunately, you'll need to open up each one until you find the one you're looking for, assuming it was made in 2000 or later. (Keep in mind Atlas does do re-runs of roadnames, so you could find that an RS-11 was made in 2005 and 1995 with the same roadname.)
AFAIK Atlas has never issued a factory-decoder-equipped RS-11 so either as suggested it's had a decoder added, or the seller really means it's "DCC Ready".