I thought I'd add this here partly because I'm starting on my little batch of 21" cars (two sets, one first run from a couple of years back and one second run) including the Excursion set. I've have had to think about what to do and how much work is involved. Also Mr Scher asked for opinions so politely.
I've had my Texas Special (first run) dome car apart for a while to install overhead and other lights in the dome section. I've learned that this is not prototypical but don't care as without extra lighting the dome is, well, a little boring. The single color moulding of the interiors also does not do justice to the details moulded-in but more on that in a minute.
I think that the signature car of the Excursion set (apart from the flag baggage car) is the City of San Francisco dome observation/lounge car. This is a photo of mine, which is inferior as a photo to others I am going to "borrow" below but illustrates what the lighting and window placements look like. The ability to produce prototypical window placements was one of the advantages of ABS bodies Lionel referred to way back when they first announced these products and in this case they achieved that much:
Note that in the forward third of the car there is no window on the right side; and on the left side there is just one. This is basically a prototypical window placement. These next two photos are John R's; top view is of the left side of this car and bottom is the same angle and right side as my photo:
The Challenger dome car has a different window placement, also prototypical, as shown in this photo from another poster, with a full set of windows for and aft and thus the interior assembly is almost fully visible:
Unfortunately the monochrome light yellow of the interiors is just too plain. Among other things it conceals such moulded-in details as are included. I'm not suggesting that Lionel could and should have included a color scheme like the real dome observation but what they took from it (like the tables with cup holder insets, just visible in my first photo) really doesn't show up in the uniform color used; here's UP's own page on this car:
This is pretty gaudy in an old Pullman Palace Car kind of way but if you want to produce even a rough representation of the real thing or maybe just put in passengers for color, it is no small job to do so on one car let alone a 7 car set (or 2 or 5 sets).
I have not opened up either of the UP dome cars yet but I know what to expect from other people's photos and working on the first run Texas Special dome car. Underneath the dome there's this odd construction (top photo UP, bottom Texas Special after being painted):
Why there's an enclosed area in the middle of this I can't figure out although it's handy for placing the power supply to the LED dome lights I want to install (as you can see the wires are already in place). This moulding is undoubtedly common to all the Lionel 21" ABS dome cars and an issue is that the uprights/partitions end up being placed directly in some of the cars' side windows, which seems odd and unsightly to say the least. You can see them in my car and a picture below (which also illustrates the hanging wire problem that fortunately I have not got):
Maybe all these upright sections were necessary as part of the moulding process - they don't support anything above them - but I reckon they can be cut away to form a new compartment, although that will require putting in details and passengers.
Anyway, where I have got to so far on the first run Texas Special dome car is shown below. I have found this car to be fairly sturdy in both the frame and body shell and it was a 5 minute job to separate them. But once you do, the interior really can't be left (a) unpopulated and (b) the same dark color throughout, so I had a go at it; no passengers yet as I know installing them will be tedious.
However, bearing in mind this is a complete fantasy car I did not feel limited in my choice of features. In particular, when I happened on a bottle of paint called "Royal Ruby" and remembered that this is supposedly a Missouri-Kansas-Texas car, I put two and two together and came up with "we're not in Kansas anymore, Toto." I know all that imagery has been hijacked in other contexts but at about the last stages of the MKT's passenger service I was still a kid watching the Wizard of Oz every Easter and so this interior got the Ruby/Emerald treatment. It includes various Oz movie and MKT historical posters and, inevitably, some for Lone Star beer:
This is not totally dissimilar to what Alex M. did with a set of first run ESE cars when they came out - if you can remember back that far and to that thread.
In the Excursion cars I am going to adopt a more orthodox approach as the UP Heritage Fleet website has lots of images of the cars in service including the interiors and their fairly plush hotel lobby-style decor (or maybe its plush saloons - but I'm guessing of course). I'll freely adapt what's there but bearing in mind that these cars can just be representations of the prototypes and not scale replicas.
But this is an awful lot of work and I can't say I am totally happy at that prospect.