Not sure what you mean by if the coaches are "all the same" or "different?" You mean what Athearn made, or something else?
I went digging in my closet and found my Railrunner set (awaiting layout completion, along with all my other rolling stock in there...). I've got a three-car set, which includes Railrunner coaches #1002, 1003, and control car 1105; plus I also have a single separate coach, #1004. The Athearn product numbers on these are 25910 and 25710, respectively. I think there may have been a few other coach numbers available out there, as I see #1011, and a 3-car set with #1009, 1010, and 1012 online too, albeit all discontinued looks like.
Good luck in your search. As you've probably discovered, finding out of production cars can be difficult, probably need to keep watching Ebay and haunting your local train shows.