Thanks, TRW. Agreed.....and great information regarding your friend's purchase from the K-line service dept.
Just FYI, the information I have about the small number of black-railed MP15's came from a retired gentleman who was the original curator of the gift shop at the Yuengling brewery. He was the person responsible for making the original Yuengling train set available. I purchased a complete original set from him many years ago. He told me about the black-railed MP15's, but even he wasn't sure of the numbers, nor did he tell me the whole story. In hindsight, I could kick myself for not spending more time with him to get all the details, but I didn't think it was too important at the time, I guess.
Anyway, I have a high level of confidence in the general story and that the black-railed MP15 I just purchased (along with 3 other cars with matching serial numbers) are legitimate. I bought them from the original owner, who had that serial number assigned to him and who got them directly from the gift shop.
I actually have a theory about how these black-railed "factory mistakes" came to be, but I'll save it for another day. Let's just say that I don't think these were really "mistakes" at all and leave it at that. For now, I'll continue my fact-finding mission. It's not like I have anything better to do! 
Thank you again for your input. Much appreciated!
