I think, but am not sure, this is am Atlas (definitely) GP38-2 (not sure about this) engine.  I'm new to the hobby and have focused more on steam engines than diesels to this point.

The paint is, in my opinion, a mess.  It looks to me like another hobbyist did a custom paint job, and not a really good one.  That said, it does have couplers, and unlike most of the old diesels I run across, it also has its horns.

Please take a close look at the pictures to determine full condition -- I've done my best to describe it, but my eyes aren't great, and like I said, I don't really know this engine.

The engine does run.  I think it is one of the best runners (along with it's sibling which is also listed for sale) I've used at low speeds, and in both directions.  At full throttle, it doesn't run as fast as many other diesel engines I've tested -- please see video.  Whether that indicates it needs a tune up or just means Atlas engineers these to run at lower speeds to mimic scale I don't know -- pretty sure this is the first Atlas I've come across.  Please see video to see it running.