Thanks guys. I'm still not sure what makes sense. Mark is correct that the Tufts book only indicates that the 1960 version of 20550 set had 2 cars, but this does not seem right. Both the 23099 and the 20550 set were cataloged with 3 cars in 1960 and the catalog specifically indicates that the 20550 included track and a transformer. It's curious that in all my years of collecting I don't remember seeing a 3-car 23099 set or a 20550 as cataloged. Gandy's picture of a 3 car set is interesting, and I noticed that an eBay vendor offers a similar 3-car "repro" insert with the 3rd car where the track would normally go. Still, I wonder why such a set would still have a transformer. Whatever the case, either the Tufts book is in error, the Deger/Greenberg book is in error, or the set was never offered as cataloged.
No wonder we Flyer collectors often seem confused!