Clem the trucks were retooled to bring them down considerably from the first release. That is why I am waiting to see how they look... if the look great I am ordering the new UP 21 in Challenger passenger set too.
Well, now you've gone and done it; I'd made a mental note to skip the Challenger cars but just had a closer look at the catalog after thinking about your comment.
This revealed some things that might be interesting while we wait and see what the current shipment of the 21" cars look like. Also it's an excuse to post some photos for possible comparison purposes.
Here's a rough copy of the 2018 catalog page for the Challenger cars:
The thing that slightly jumped out at me in the illustration is that it shows what are (for want of a better word) bellows-type diaphragms between the cars and centered at the end of each car. Now I know better than to rely on Lionel catalog illustrations for accuracy and this one is pretty plainly not representative of any detailed plans because none of the cars is named and I can't even make out road numbers.
But if you look back at the 2016 illustration for the UP Excursion car sets actually there's no diaphragm detail at all, which made me assume at the time that Lionel was going to use the full-width diaphragms of the original 21" ABS cars issue that goes with the close coupling mechanism:
Not that it matters too much to me but I know which is prototypical (the first illustration). However it would matter if you wanted to combine the sets and they do indeed have different details in this respect and possibly different coupling mechanisms. I can't believe that there will be a difference but we won't know until somebody shows us a sample of the cars now shipping.
This also made me look again at what I already have in the way of UP passenger cars and here's an example of two dome cars:
The bottom one is what I think is one of Lionel's last 18" aluminum UP cars; the top one from an MTH DAP set. Both were issued several years back and have diaphragms in the center of each end. The photo doesn't do either of them justice.
The other thing in the 2018 catalog that belatedly caught my eye is that at around p. 46 some other 21" cars are listed as having passenger figures (!) although that's not included in the Challenger cars description. There's also a StationSounds full-length dome car, which I assume will have dining car announcements. There's been no fanfare about this although I think it's completely new - unfortunately not in a RR I model.