The red engine, the yellow passenger car and the two flat cars are from the original Gilbert American Flyer Circus train set.
The Circus Train 5002T set was offered in 1950 and used the streamlined 4-6-2 pacific to pull it, the engine had no sound and no whistle, but looked really great in red (The other version of the postwar streamlined steamer was blue and decorated for the B & O and called "The Royal Blue"). The red locomotive had "American Flyer Circus' in yellow on the locomotive body above the white side-walled driving wheels, the tender also had "American Flyer Circus" in yellow painted on it. The yellow New Haven style streamline passenger car had "American Flyer Trains" in red above the windows and a big, bold "Worlds Greatest Show !!" in red centered below the windows. The two yellow flat cars each had a green tractor truck pulling two four wheeled barred cages with a single animal in each cage. The items on the flat cars in your photos are not original to the set. However, with what you have it is still a really cool set and it probably wouldn't be too hard to get the steamer to run again. There used to be replacements on the aftermarket for the tractor trailer trucks and the four cages... they were not identical to the originals, but they were close. I have no idea whether an aftermarket AF parts person has any of these items available currently, but at least now you know they existed.
Flyer collectors have always regarded this as a desirable set to have. This just a quick overview, but I hope it helps.