Concerning the Clinchfield and the Delaware & Hudson Challengers. Like the Rio Grande, Clinchfield had its own design of Challengers, which were very similar to those of the Delaware & Hudson. Clinchfield then acquired the UP designed Challengers from Rio Grande as well, as it saw an opportunity to get six engines at a bargain price.
So, the biggest difference is in the tenders, but the D&H 4-6-6-4's resemble CRR's own design and not the UP one. But, they are fairly close in looks.
They're not my cup of tea, but I am extremely pleased that these are a great success, hopefully it means we'll see a lot more N scale steam.