Here are some photos of the UP Anniversary cars. I shot just the dome and observation. I bought this set several years ago, just to get the cars. They were only available as part of this set, as you know. There are seven cars, which is nice; the set includes a sound diner. These cars are outstanding, the finest 15" cars in O gauge, IMHO. Lionel made this series in a number of roads, but not too many. I also have the Santa Fe Super Chief set that was in this series, which is gorgeous. Personally, I'm just running mostly 15" cars now, and selling those 18" I have - they're just too big for my home layout.
You're correct, the Alcos looked too large for the cars, and I sold them. Lionel wasn't yet making 18" Aluminum streamlined cars at the time this Anniversary set came out. In addition, the original Anniversary set had Alcos (the small O-27 FAs), and Lionel only had the scale Alco PAs to put with this set, so that's how it came to be.
I have these cars behind some Lionel UP GP-30s at present, but they'll ride behind the new Legacy Lionmaster UP grey/yellow striped Challenger as soon as it arrives. They look great behind Lionmaster Challengers, or F-3s, Geeps, etc., but the PAs do seem too large (although most visitors wouldn't notice).
Lionel also made a different series of 15" aluminum cars with interiors and figures, including a Union Pacific version, but these should not be confused with the set shown below. This other passenger car series does not have anywhere near the detail - they are basically traditional 15" aluminum sets, except with interiors and figures. Neither the interiors nor the figures have the level of detail nor are they painted in different colors, and they don't have the detailed trucks or add-on details, etc.
This series of detailed cars can be quickly visually distinguished from the other series of 15" aluminum Lionel cars I mentioned above, because they have highly detailed diecast sprung trucks. They also have the moveable rubber vestibules, add-on metal rooftop vents, handholds and antennas, painted multi-color interiors (including toilets with fixtures), better detailed figures, etc. All in all, they're great cars.