Andy,
I believe I've seen a five-pack set in a hobby shop here in Tulsa, OK. If I remember right, they are red and silver. Four assorted cars (baggage, diner, coach - not sure exactlly what combination) and an observation car.
The railroad specialist at the shop is a Rock Island fan, so if that set doesn't suit your needs he might have some other ideas. I'll get more details on the set above and pick his brain today or tomorrow. Get back to you as soon as I can.