Hello David,
As you know, Scott does not give his sets catalogue numbers, so I cannot easily discern which were the two add ons.
I received them all on June 6, 2009 directly from GGD; so I'm guessing the last two on the list were the add ons.
The order in which I recorded them:
1. RPO
2. Baggage
3. Hamilton Fish
4. Samuel J Tildon (Parlor)
5. Dewitt Clinton (Dining)
6. Franklin D Roosevelt (Observation)
7. Nathan L Miller (Parlor)
8. William L Marcy
They have a patina which differentiates them from all of the aluminum and silver cars of the SF and the CZ. As with most GGD cars they are heavy, and I could not pull them with the brass 3rd rail J3 Hudson.
I don't have immediate access to the cars or any pictures, but I could retrieve them from storage.
I don't see them auctioned very often, and I have no recollection of seeing any add on two packs.