Well, the plot thickens. It's possible that whatever TTM folks you talked to, may (or may not) have been dyed-in-the-wool toy train enthusiasts and historians, but rather some employees that merely direct traffic in and around the museum, run the cash register, etc., etc.
For right now, I'm just guessing that these were most likely a special run of passenger cars done at some point in time exclusively for the TTM by K-Line trains. I know that K-Line did quite a few small special runs of cars for all kinds of folks and events over the years. For instance, I have a couple of K-Line O-gauge cabooses decorated for the Topeka Railroad Days, Labor Day Weekend 1987. I imagine these cars were supplied to the show organizers to sell at the show as collectible money raisers for the event. I seriously doubt if any special run cars were ever cataloged by K-Line, which would also explain why info. is fairly sparse to almost non-existent on them.
In any case, maybe the TCA (Train Collector's Association), TTOS (Toy Train Operating Society) or Roger Carp @ CTT (Classic Toy Trains magazine) could be other good sources to contact and see if they know something about these passenger cars.
And as a last resort, when you manage to obtain the entire set, take some good, clear pictures of everything and post them here to see if something doesn't ring a bell with anyone.
Hope this helps a little more. Kinda' got my curiosity up a little now, too.